I have and ride a onewheel but never skateboarded. It really is a different balance; one-wheeling hasn't helped me a bit for skateboarding skills either.

My take on the safety bit is I would never ride the one-wheel at the top speed the device supports, and would never ride without a helmet or on a road with traffic. It can and will dump you. I've had the battery-out dump (when I knowingly experimented after its low power warnings). Basically if you go faster than you can run out/step and roll out from you're inviting yourself trouble.