The biggest problem with these guys is that they like to pick up loose frags and carry them around, dropping them in inconvenient location. Between my urchin and my brittle star (who always drags things under a rock), I couldn't keep a frag that wasn't nailed down.
They won't destroy live rock. Depending on how large your aquarium is, the coralline should have plenty of opportunity to grow back.
Long-spine urchins get really big, and could cause a problem for bubble corals or other soft-fleshed LPS, I suppose.