There are 'reef safe' urchins out there...they just come with other side effects most reef owners don't want. Urchins are like tanks and a new teenage driver behind the wheel. This always gets our brand new frags knocked over into the sandbed...usually when you are at work and it is already too late for it by the time you get home.
The most popular reef safe urchins are the tuxedo urchins. The drawback to these guys is that they 'camouflage' themselves with whatever is in your tank...be it a rock, a snail, your prized bubblegum chalice frag....whatever.
So, even though urchins have the reef safe tag along with them, no urchin is worth the price of the corals we put in them IMO.