Very true.
If you're not sure about a site's accessibility, see if anyone you know who uses a wheelchair is willing to go with you on a test visit. Generalising greatly, if a wheelchair can get across terrain, it's accessible to all.
3 wheelers are easier for stony tracks.
There's also the element of how much assistance would be required. Possible grading could say be based on one end of the scale being accessible to electric wheelchairs on their own, the other end meaning that carrying (and thus more people) would be needed.
Independance vs enabling kinda thing.