I am being using walk assist mainly on mountains. I found that a value of PWM duty cycle we set for walk assist on a flat terrain is very different from the one on mountains -- we need a very high value on mountains and if we use that value on flat, by mistake or just because we need first to try to see if it is ok or not, the bicycle will simple go off very fast and that is dangerous.
When I had to use walk assist on a steep stairs (or big rock), because on uneven of the stairs, I had wheel running very fast sometimes to just after run at the speed I need -- when the wheel loses traction momentarily, will run at very fast dangerous speed. And when it gains traction again, the energy on the wheel from being rotating such fast, gives a strong pulse of force forwards and all this makes hard and dangerous to keep on control of the bicycle.
I think walk assist is a must to have and I prefer to have as it is however I know it is dangerous and for that reasons I don't want my 9 years old son to use it.
I think it must have a speed and power controllers as also soft start. As user, I want to configure the speed and max power(currently max power is like 900 watts on walk assist!!). For steep hills and dangerous terrain I would go with low speed like 4 or 3 km/h. For the power I would configure some low value like max of 200 watts and usage will depend on the weight of the bicycle, if I pull myself on the handle bar and the inclination.