It seems to make up the relatively steep hill I live on, but I've never seen him use it to go down the hill. Guessing it gets bit sketchy fighting gravity.
The big risk is going too fast: because the wheels use balance (forwards/backwards) to know whether to accelerate or slow down, if the wheel goes beyond a certain speed it tends to behave somewhat oddly, some models will lock and stop, others will "freewheel", either way the user will likely end up eating pavement.
IIRC the latter is the behaviour of the interviewee's rockwheel, it starts beeping (in several increments) when closing in to maximum speed, then the systems just shut down and you get a completely unmanaged unicycle (without electronic stabilisation). Other wheels will try to forcibly rebalance the user, but again it's fairly easy to defeat, not much the wheel can do if you just lean forward despite its best effort… and you eat pavement again.
Aside from that, acquaintances who've tried/used them tell me they're fairly fast to learn and very enjoyable and convenient.
Sure. But there is a higher risk on losing control on downhills, and if you do the only brake is to bail.
The government apparently pledged to make Segways (and then presumably all these self-balancing 2 wheel electric scooters) legal but never actually did.
[1]: http://www.legalisesegways.co.uk/ [2]: https://www.gov.uk/electric-bike-rules
https://www.gov.uk/mobility-scooters-and-powered-wheelchairs...
In particular as you need to register them which seems pointless and antiquated.