I'll negate that a bit. I had a diadema urchin, and once it grew large, it munched on some of my SPS (a few small bites, here and there.) (I think it was confused between encrusting SPS and coralline.) Todd W. got it from me since he didn't have SPS...and it ate an entire one of his softies.
I have two rock urchins that I've had for about a year. They're supposedly not reef safe as they'll eat softies, but I haven't had that problem yet.