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.