I own and ride a Onewheel Pint (17mph max) and found it limiting, so moved up to an EUC (electric unicycle) that goes 30mph. I always wear a helmet and come from an extensive background riding BMX on ramps and jumps, skateboarding, snowboarding, and mountain biking.

I agree with some other comments here that there is a learning curve and these are relatively dangerous for new riders, just like a skateboard is dangerous. But I do not think these are inherently more dangerous than a normal skateboard or bike. It's just that the self balancing and electronic assistance can give an unearned sense of confidence where the rider is less cautious than they should be.