You lean back as hard as you can and the regen breaking stops you. It works pretty well actually, if you look up "EUC braking test" on YouTube you're likely to find some videos that will give an idea of their braking performance.
I think the niche of "personal vehicle that you can take on the metro" is better filled by PEVs. Shrinking down a bike's drivetrain size is harder than just putting together a single wheel, some circuit boards, batteries, and an electric motor to make an electric unicycle.
A big advantage EUCs have is you don't have to bother with folding like a bike, just hop off, grab the trolley handle and walk on.
Electric scooters have similar advantages but they have to compromise range, performance, and compactness for their ease of learning.