This article is from the Unicycling FAQ, by Beirne Konarski with numerous contributions by others.
As dangerous as you want it to be. The vast majority of falls leave the
rider on their feet, as dismounting is a skill learned while learning to
ride. Most falls occur for just a few reasons: excessive speed, exhaustion,
lack of attention to the road, and learning new tricks. The rider can
control all of these factors to their desired comfort and safety level.