The difference is that when most things fail (something that will happen), they do not result in instant deceleration. When your iPhone experiences a power failure it usually just doesn't turn back on. If a car experiences a power failure the brakes still work. Even if the brakes DO fail, you can still at least try to come to a safe stop—and it's possible you can.
When the power fails on a OneWheel there is almost no possibility of a safe stop. You are getting launched.
Yes, agree with you both - probability * severity. The severity is high (unless you know how to run off the board - that lowers the severity quite a bit). The probability is unfortunately unknown.
I think the severity of the event is also important. A Onewheel requires power for stability (it's an inverse pendulum), and as a result does not fail-safe. An occasional motor failure on an e-bike would be a non-event, as it fails to a safe state. An occasional brake failure would be significant.
When the power fails on a OneWheel there is almost no possibility of a safe stop. You are getting launched.