Unis are small, slow and safe.
As a teenager I would go out for a ride, put it on my shoulder for hikes and then ride it back by bus.
So when you take the novelty off (and as a rider, it vanishes quickly) it's just a very fun and convenient bike
But they are useful as drag brakes! I run a disc brake on my mountain unicycle, and really it's to make super-steep downhill segments feasible (at least without requiring a tremendous amount of back pedal pressure). I choose a brake used for trials biking with a long throw range.
I have a rim brake on my 36" unicycle. In that case it is helpful to reduce stopping distance, in part because the crank-to-wheel ratio is bonkers, but it's still primarily used as a drag brake on long downhills.
But yeah, on a mountain/street unicycle, your feet aren't that far off the ground, and you can't really go faster than sprinting speed. The big risk is getting tangled up in a weird way on the mountain.
36" distance unicycles can go faster than sprinting speed. Bailing unexpectedly on a fast 36" results in a few galloping steps and hopefully not a trip and slide...
Fun little unicycling factoid: "UPD" stands for Unplanned Dismount, and really is what tends to happen more than actual crashes. You realize your balance envelope is blown and there's no hope of making the line you're trying to hit, so might as well just dismount gracefully while you still can.