I gotta hand it to anyone who takes a unicycle Downtown. That guy isn't me, but I've done it a few times.
Unicycles do not categorize well. You can't ride them very well in traffic, as their top speed (for a standard 24" wheel, anyway) is only a jogging pace. Probably you shouldn't ride them on the sidewalk because they're technically considered pedalcycles. Probably you should be real careful with them on the sidewalk, aside from legal status, because you tend to scare the hell out of everyone.
Four years of unicycling across the SUNY Geneseo campus every day, I never once touched another person, so there is nothing inherently unsafe about it. You can do as much damage to people swinging a briefcase or with a laptop slung over your shoulder.