The main problem I can see people having isn't to do with the controller but sourcing the accessibility switches themselves isn't easy, most can be obtained by healthcare professionals such as Occupational Therapists but as they can be expensive (especiallly the ultra sensitive ones I need) most health authorities will only provide a single switch at a time and most don't see gaming as a necessity to daily life.
There are charities like the wonderful SpecialEffect who provide equipment and advice to disabled gamers who can obtain the switches but obviously can't help everyone as they don't have infinite resources.
I recently managed to buy some of the ultra sensitive switches myself at a reasonable price (£22 each) but I had to import them from the US and the import fees were ridiculous,, three switches ended up costing me around £115. Tried looking at UK suppliers but the switches I needed were £60 each so importing was the cheapest option available.
These are the ones I need as I have very little finger mobility:
http://www.inclusive.co.uk/tash-microlight-switch-p2608The ones I got were a cheaper alternative unavoidable in the UK but aren't as durable or as easy to position as the above.