Self balancing and motor output have an inverse relationship. As the motor works harder, the self balancing becomes weaker. So at 21mph, the "warning" you're expecting is pretty much too brief for your brain to notice, there simply will be no time to react. The moment your balance stresses the motor too much(at that speed), the nose is already slamming into the ground.
I don't mind the "punishment" for certain bad moves being wiping out. But nose diving as the "punishment" for going too fast appears to be extremely dangerous.
...it's a one wheeled vehicle, how else could it respond when the limits are exceeded? I don't mean to sound condescending, but these are fairly basic physics we're dealing with here.