The motor is holding the nose up. When you lose power, the nose immediately dives into the ground, stopping the onewheel, but not the rider.

If the rider is actively balancing then it doesn’t need to immediately nosedive. In reality that’s the usual effect, but keeping a low center of gravity and trying to keep it balanced go a long way toward a smoother slide/roll instead of a faceplant.

The rider leans forward to accelerate the OneWheel, so they are not "actively balancing" - the OneWheel self-balances "against" their lean. Until the motor gives out, that is.

Simply carving , which is how you turn, requires some balancing by the rider. Going fast over anything but ideal terrain also requires active balancing. If you ride one in any capacity beyond a few mph in a parking lot, you’ll feel it in your legs.

I owned one!

That's really unbelievable - an unacceptable mode of failure. How are they not in jail or sued to oblivion? What engineer put their name on it?

I suspect it has more to do with the active balancing on the thing stopping from one moment to the other. You’d just tip forward before you can react.