I have had urchins in my tank continousely for at least 40 years and never had a problem. I don't remember when I got this guy but it was years ago and he is about 6" wide.
They cover some reefs in the sea like you can see here in Hawaii.
They may move small things around.
I used to have an urchin collecting business and once I had 24 of them in there at once. They didn't hurt anything but they all decided to spawn at once. That was a mess.