I don't ride these but I and many friends ride electric unicycles, EUCs. There's a known safety concern: if you're riding at the maximum supported speed, and yo

I don't ride these but I and many friends ride electric unicycles, EUCs. There's a known safety concern: if you're riding at the maximum supported speed, and you lean forward, you're going to fall forward because the device can not speed up to catch you. Similarly, if you lean back to brake but are leaning too far for how quickly the machine can stop, you will fall backwards. At high speeds this is catastrophic. You can also draw too much power due to things like incline, music, etc. OneWheels would presumably have the same limitation. EUCs have default warnings at high speeds, but you can disable or change these warnings (making them higher or lower). You also need to monitor your battery health; I wouldn't trust an old battery. Ultimately the rider determines the risk. I'm the only rider I know that wears a helmet. Many friends or acquaintances have broken their legs badly or been hit by multiple vehicles. I ride slower and safer, generally, but even I've done some minor damage to myself. I'm inclined to get rid of the EUC in favor of a scooter or ebike. Two points of contact with the ground is so much safer.