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.