Yesterday a couple miles down the road from me 2 guys rolled a RZR. One is dead, the other was flown to Boise and is critical. Neither was wearing a helmet. UTV's aren't invincible.
4 legged critters can hurt you just as bad. Still, I don't like to ride anything where if you let go the steering it falls over.
I don't know about seatbelts but they weren't wearing helmets.
One of my hunting partners was banged up pretty bad a year or so ago when his horse shied a bit and stepped over a bank. They were going around a group of boy scouts on bicycles. The scouts had stopped to let them go by but the horse was a little antsy. The boys got a chance to use their 1st aid training while one of them rode on out to the road to call for help. They flew him out. He had to wear a neck brace for a long time. He'd had a double hip replacement some years ago and they were worried that it was messed up but it healed ok. He was able to elk hunt with us that fall although he couldn't carry anything heavier than his rifle.
I'm putting a harness bar and some 4 point harness' on my Rhino. You can never have too much protection on these rigs. I don't usually wear a helmet either when riding the Foreman or Rhino but that might change this year.