IME, reef safe with caution. I have never had one eat corals as some people have experienced. They do strip your live rock of algae including coralline algae(which sucks). They also knock over anything that isn't strongly attached which could cause problems hence the "with caution".
If I remember clearly, my experience with the pencil urchin was that he was a bulldozer. I find the long spine black urchins to be very gentle, clean algae off your rocks like mad, and don't carry anything around. Not as cool looking, but my overall fav for a reef tank.