They're reef safe with caution in my experience. Getting a spine in your hand is one of the most painful things I've had happen in a tank, short of a queen trigger bite and a rabbitfish venom stab. So don't work in your tank with the lights out! They also bulldoze frags (SPS, soft coral) if they're not secured well and can graze the edges of them if they don't have enough area to eat or move through.
I kept a hitchhiker one in a 90g anemone tank, but it outgrew that even after a few years.
So big FO system with no triggers would be a great addition. Maybe someone setting up a new tank that needs algae control. Or someone curing live rock in a huge bin with proper heating and water parameters.