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.
The big problem with piping natural gas to buildings is that the miles of pipes that deliver the gas leak. Gas leaking is a real issue since an equal amount of un-burnt methane has 25x the climate change potential as CO2. Even if the power plants supplying electric heat pumps/radiators is natural gas, the shorter supply chain (only to power plant and not to every building in the city) go some way to offsetting extra emissions. Then you have the obvious benefit that once the NYC grid transitions to renewables there won't be as many natural gas ovens and heaters that need to be replaced.
The commenters pointing out that natural gas stoves don't have a significant effect on emissions are right, but once a building gets a gas hookup installing natural gas heaters and water heaters becomes more attractive and economical. That's something we don't want to be incentivizing
Climate town recently released a new video talking about this issue as well as the decades of marketing and lobbying the natural gas industry has paid for to neuter or outright kill legislation like this.