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I've tested this. The fastest I could get a Voi scooter in Germany to go downhill was 27 km/h. They're limited to 20 km/h on level ground and gradually start braking if they exceed that speed downhill.

It might be possible to find a hill steep enough to overcome the braking force, or one that results in a strong enough braking force to make the scooter difficult to control, but it should be obvious to most people that attempting such a thing is dangerous.

All the ones in Sweden use the electric motor to govern the speed downhill, limited to 20 km/h.

Why not? The same brushless motors could be also used to brake the scooter could they not?

I haven't run the numbers but at that size I would expect them to get very, very warm.

Couldn’t they just do a controlled spark at high enough voltage?

1. Regen braking

Sure, up to a certain grade. But regen braking isn't working down hills like the ones in SF.