I've only seen one unicyclist on the road before and that was on the A4 in London of all places.
Don't think they look particularly safe to ride on roads though? It's basically a very wobbly fixed gear bike with only one wheel, no brakes and no steering wheel right? Even skateboards/longboards and rollerblades are probably safer on the roads.
I'm not sure there's any reason why an experienced unicyclist would be more unsafe on the road than a 2 wheeled cyclist, with the possible exception of speed (depending on your view, as being discussed in a different thread), and any confusion other vehicles may have simply by seeing a unicycle.
Someone riding a unicycle on the road is probably using a 29" or 36" wheel, which gives them a good extra "head" of height/visibility. Once you've mastered riding on one wheel, the number of wheels is pretty much irrelevant. In fact, I am much, much more manoeuvrable on a unicycle than I am on a bike.
Brakes-wise, as unicycles are fixed geared, you brake with your legs, so it's not correct to stay they don't have brakes, they do, and they're very effective. In addition, disk (and rim) brakes are not uncommon on road and mountain unicycles... Although they're usually used to "flatten out the downhills" rather than for stopping.
I hope the rider recovers quickly and completely.