ALL Indoor/Outdoor "Full Time Use" Powerchairs should be ALL TERRAIN capable.
There are simply no downsides!

Its actually pretty easy to make a true dual personality powerchair! And have a truly usable "do everything" power wheelchair that would suit most Paraplegic / Tetraplegic users because as a "paraplegic end user" and as a pissed off engineer  I built one in a bedroom in about 21 days...

Would you believe that of all the hundreds of PowerChairs available NON are really capable of doing both jobs well.  (As far as I know)  Why not? because its is obviously what every full time busy wheelchair/powerchair user actually needs!.

I will break down the requirements for a SIMPLE DO ALL indoor / outdoor powerchair into easy to understand parts...

• Short. Around 40 to 42 inches maximum TOTAL length including any footplate's and the users size 11 feet!   Longer simply hinders indoor capability. Any other quoted length (like minus the footplate, or ignoring rear anti tip or extended rear caster wheels is absolutely meaningless other than for transporting it empty and fancy figures for the marketing department. Ignore brochures and MEASURE your future chairs! Mines a true 40 inches.
  
  And this measurement it must include any anti tip wheels or rear casters rotated to the longest position. (maximum length).  My home built one is 44 inches INCLUDING my size 11 feet as I can't drive about without them!  (or 40 inches without my feet) and I could make that 4 inches shorter still with a little work - there's room.)  Manufacturers do quote these specs and they are extremely important, but often on a misleading way. Every inch really does matter indoors.
  
  
• Narrow. Total Width 25  to 25.5 inches Maximum.  That is a usable indoor figure for good manoeuvrability and easy comfortable door clearance. Its not about whether it will fit through a doorway but if it EASILY does so. And have lots of spare room around furniture etc. Works in vans, pubs and other peoples houses too. This figure must INCLUDE an 18 inch wide cushion fitted! for e.g. (if that's  the size you use).  Substitute your own size here.
  
  Because quoting less width as most manufacturers do, with a "narrow" cushion is plain cheating, misleading  and is incorrect in actual use and for all real comparison purposes.  Mine on my "do everything chair" is 25.25 inches wide at the widest point even with my 18 inch cushion in place. This is important because on most powerchairs the arm rests are adjustable in and out to accommodate different sized cushions. 
  
  Wheelchair width is VERY important and the chair needs to be as narrow as humanly possible. Certainly no more than your body width Seated. See Powerchair Width for some typical bad examples.

• NO CORNERS! Swing away footrests, rear casters that always rotate and (hit stuff) form corners at both ends! Indoors this is just a definite NO! It means manoeuvrability and freedom around furniture/bathrooms etc is hindered drastically.  Every inch or part of an inch matters a lot here!
   
• Programming -  NO or very little delays. No "accelerations" either in turn acceleration, turn decelerations, or anywhere else at both slow and fast speeds. These things make it like driving your car with the controls connected by rubber bands... This is the greatest problem with every powerchair I ever test or use. TURN IT ALL OFF!!!
  
  Make the controls absolutely direct. Like a model car or plane. Or your car. Then it goes where I want it to go, when I want it to go!  Suddenly its CONTROLLABLE and accurate. End user programmers do not allow this OEM level programming to be corrected.
  
  So either supply a OEM style programming tool with each chair for users that want one or program it correctly to begin with.

Well basically that covers the indoor part pretty well. Stick to the above and it will be great Indoors Only!  This first indoor section above has just ruled out 95 percent of all current powerchairs. Even the small "indoor only" old people's cheap (and ALL the NHS "wheelchair Services" ) motorised deck chair ones... And that's before we go to the shops into the big wide world!

• Be fast enough to cross a busy 4 lane junction without getting you killed, or get you home in winter before you actually die of exposure. A FOUR MPH or less powerchair here is a real JOKE. its really and quite seriously unsafe. People crossing busy junctions often need a spurt of speed just to make it across safely!  The traffic wont wait! 6 MPH or more is just about acceptable here and if you are heavy is really the upper limit too due to current used battery technology. (Although better battery technology has existed for years. we are talking about safe lithium technology - lithium ion phosphate)
  
  But see "range" and the other problems with too much speed. 6 MPH is better than 8 or more if you are heavy, live in a hilly area, or intend to go off road much.  You cant fight physics and current batteries are not capable of more all day long. Its a compromise.  If you weigh about 7 stone you may get away with an 8mph powerchair.  Group 24 70 Ah batteries ARE THE BARE MINIMUM for outdoor usage. Less is completely useless.
  
  
• Be capable of driving "comfortably" on our modern pavements and roads.   Never mind off road!  ALL powerchairs that are designed to go on our streets rather than to a shopping centre occasionally NEED some off road capability.
  
  Our streets are really bumpy, uneven and damaged. Normal powerchairs rattle you about either causing or aggravating pressure sore issues due to the sheer forces and muscle spasm problems for many people. And spine and other problems as all the "shock loads" hit you.. Just watch someone in a typical wheelchair getting rattled around on our pavements and uneven roads and you may get the idea.
  
  I have tested and tried a great many so called outdoor power wheelchairs. Including many with "suspension" for what its worth. They all hurt my back and rattle me about like a pea on a drum and cause me severe muscle spasm problems. The manufacturers don't have to sit in these things obviously. Powerchair suspension is ineffective. Too little travel, so far too hard.

• The problem here is that our streets and pavements in towns and cities are FAR from flat and smooth!  This is an extreme example but in reality most of its a bit "rough" compared to shopping centres, hospital corridors, and the wheelchair manufacturers factory floor or car park where they obviously "test" them...
  
  Some places are unbelievably bad though.  And current powerchair suspension has hardly any "travel" (an inch or so if you are lucky) compared to say a car or motorcycle with 6 to 12 inches or more. Those tiny hard 3 inch wide or less standard "grey mobility tyres" (even worse with foam, "green", solids! ) just transfer every shock and bump straight through to the occupant. My back cannot stand any more!  You always spot that bit of broken road surface or next bump or drop just to late to slow down enough to avoid it! It hurts unless you go far too slowly...
  
  At the speeds we travel at in a powerchair, the suspension that we get just does not work well.* It cant, its more important to have lots of "give" in the fat soft tyres. (*Other than for marketing purposes and sales in the showroom!)
  
  Big fat soft (very low pressure) balloon tyres cure the vibration and road shocks almost completely, bumps and uneven pavements are beautifully smooth, Even cobbles are smooth like they don't exist. It works like no suspension ever can at low speed as the tyres just deform around everything.  I have two identical powerchairs with one crucial difference. One has suspension and typical 3.50 x 8 (14 inch tyres) and the other has NO suspension and big fat balloon off road tyres. Guess which I use! The difference is quite remarkable. 
  
  See THESE two next images, BOTH CHAIRS ARE THE SAME narrow 25.25 inch width!  Fat tyres should not mean wider chairs unless the designer has no clue.
  
  So SMOOTH riding big fat very low pressure (6.5psi) balloon sized tyres are also essential requirements for a smooth ride outdoors on our streets.
  
  ESSENTIAL equipment - once you have experienced the difference!. Mine are black. But the wheelchair industry could get them moulded in grey if you prefer that. (I actually don't!) Tyres
  
  First time I tried my freshly built powerchair I was amazed how good they REALLY were. So tyres like these are very important for any decent outdoor powerchair. You actually get a sensibly smooth ride but please remember the width issue indoors. (See above) 
  
  You CAN have seriously fat tyres like mine (6 inches wide with huge sidewalls and "floatation") and still have a narrow 25 inch wide indoor powerchair because because I did it already with my own home built powerchair and with full size batteries. 
  
  So there's no real excuse for ALL powerchairs not also having a better ride and be capable in snow, off road, on the beach too.
  
  
• Typical outdoor obstacles. Without ever actually going "off road" or "all terrain" you come across things like this. Its actually MUCH worse than it looks here with some very difficult small steps and angles. The photo does not do it justice. The bottom is about 10 inches below where the lamp post is. The yellow line near the left of this picture is about 11.5  inches above the lowest part. This sort of thing is common in towns and cities. God forbid you live in the country!  Or worse IN the country! Or a farm etc...
  
  
• Mid drive indoor/outdoor powerchairs with a set of casters sticking out at each end cannot get across this typical "access ramp" onto the road below to cross it at all.   At least non of the ones I have tried.
  
  It seems that they have insufficient travel on their overly complex heavy caster "suspension" systems. I know, I tried a few! They get stuck with the drive wheels not in contact, or just about in contact but with no grip!. Or one not in contact and so the chair turns instead of getting "over" this typical roadside problem.  Those chairs are ok with small curbs on nice flat surfaces but this, and any serious off road obstacles has them and their tiny caster wheels in trouble!

Mid drive indoor/outdoor powerchairs with a set of casters sticking out at each end cannot get across this typical "access ramp" onto the road to cross at all.   At least non of the ones I have tried.

It seems that they have insufficient travel on their overly complex heavy caster "suspension" systems. I know, I tried a few! They get stuck with the drive wheels not in contact, or just about in contact but with no grip!. Or one not in contact and so the chair turns instead of getting "over" this typical roadside problem.

They usually have to  use smallish casters too, so as to minimise the impact of having four of the things stuck in every corner when indoors. And they jam and get caught in all kinds of obstacles since they are generally much too small for outdoor usage. 10 inch casters are really a requirement outdoors. With a rear drive chair their is just 2 of them and they don't get in the way because your feet stick out further than they do. 

Sensible sized tyres for a front caster would be 3.00 x 4 as used on my chair. Wide enough to not sink in sand/snow easily, big enough outside diameter (10 inches) to roll over big pot holes and other obstacles outdoors. Tyres

You think this ramp is unusual? Its not. There are many much worse ones, and all kinds of other issues that we have to contend with and negotiate in the real world many times per day!

The small casters typically used on mid drive powerchairs also get "stuck" on the ridges (layers of old road where they resurfaced). So since this example is pretty mild compared to some of the obstacles around here I cannot sensibly use any mid drive powerchairs as they just don't have the capability to get past things like this that I come across in the real world, which just so happens to be full of them.

In my own fat balloon tyred powerchair I could drive over it slowly as my anti tip wheels are tiny, high up and short. So they don't touch anything. And it has big 10 inch front casters so they ride over the "steps" onto the road very easily. But with the very rearward C of G that I prefer, I MAY JUST tip out of the back!   (I don't actually, I have tried it, but it is a close run thing)

So instead I just approach faster at say 2mph and then a quick "wheelie" while going down the slope with a touch of extra power (just as a manual wheelchair user would do) and then it sort of lands smoothly while moving forwards onto the road as I cross the yellow lines with the rear wheels.

Its easier to do than to describe. And its really smooth and natural. It requires some power (and some proper control system programming) and some expertise and commitment but with those big soft balloon tyres its a smooth and bump free ride and doesn't really even slow you down. You need some "feel" and some control, that stock powerchairs just don't have.  Why not? Manual wheelchair users demand this balance and control? Why should powerchairs be any different?