They're reef safe and IMO are more beneficial than not. They live in the sandbed, eat uneaten food and prevent detritus from building up.
The cons are they eat creatures that live in the sand that may be beneficial to the tank. The tank may also not have enough for the starfish to eat so it then will starve to death. Some people have also reported that theirs just disappeared after awhile. Whether from starvation or what, who knows.
So I think they're beneficial if your tank has enough for them to eat.