When going downhill, any bumps which result in severe movement in the frame that bring the rear wheel closer to the saddle might be problematic, even without full suspension. Hard to say. I’m a road bike guy but I’ve definitely seen some forms of non-full suspension when working on mtbs which would result in that kind of movement.

I think the tyres would limit the power output most. Probably quite annoying to have no rear mudguard in anything but the driest conditions. You’d be spraying a lot of crap up at the motor (and probably your back).

Attaching and detaching it looks relatively simple, though - removing it for downhills doesn’t look arduous. You could probably 3d print a clip for the frame.

Given the likely reduced power, and that the spec seems to be saying ‘500w peak, but 250w is sensible’ - probably up to the user’s needs whether the hassle is worth the extra watts per kilo. For those without mobility issues or longterm injuries, it’s usually best to optimise the rider rather than the bike for that.

I feel like off-road climbing would necessitate really wide, sensitive speed variance that this motor is likely not great at supporting. It strikes me that you’d want more direct pedal feedback.

Really fun bike design problem.