UL supports retailers and manufacturers by offering electrical and fire-safety testing and certification under UL 2272. This standard evaluates the safety of the electrical drive train system and battery and charger system combinations of hoverboards and other 1-wheel, 2-wheel, x-wheel (which includes hoverboards).
I would say to not buy anything that lacks UL2272 certification, but out of all electric unicycles available, only one has been certified. Be aware of the risk: NYC: King Song 16S Electric Unicycle Explodes In Fire. EUC makers will eventually have to comply or their products will be banned from entering Singapore. And then everywhere else.
Veteran electric skateboard reviewer Jay Boston gets his hand on an electric unicycle!
Riding an EUC is a distinct skill. Only practice makes your brain adjust to it. Like riding a two-wheeled bike as a kid. And since hardly anyone can recall how long that took, let me try a different memory tack: How long did it take to master a bicycle wheelie?