After all, when you brake hard with a car or motorbike or bicycle, it's the front wheel, well in front of the centre of mass, that does almost all the work.
In a single wheel vehicle, the wheel well in front of the centre of mass.... isn't there. In a unicycle that means you're flying off the front (it's even worse in a monowheel) so you'd better not be going faster than a man can run.
your stopping speed is limited by how fast it can decelerate you, which is essentially the same as how fast it can accelerate you. it's not super fast to stop, to be fair, but neither is a bicycle going at high speed (though I'd expect the bike to be a little bit quicker to stop).
Leaning back is not an option once the motor stops pushing the wheel towards you - it’ll roll and then tumble to a stop much more quickly than air resistance slows you down, so you’ll fly off the front.
I’d like for there to be a magic solution to this problem since the size of an EUC is very attractive. Rolling to a safe stop on my electric scooter is preferable in the meantime.
either way, safety gear safety gear safety gear. especially since basically all these electric personal mobility enhancing whatsits are horrifyingly injury-prone compared to anything the size of a bike or larger.