I would say they are totally reef safe as far as eating or picking at corals, I do have some other concerns listed below though. They spend 90% of their time just burried in the sand. They never bothered with any of my other livestock or cleanup crew. When they are out they are pretty cool to watch.
Having a couple months of first hand experience with them my conerns woud be :
1. The tend to kick up a lot of sand when they move around. In a bigger tank this may not be as much of a concern but in my 90 whenever they decided to go "hunting" for food they cause a lot of sand particulate in the water. (part of the reason I want to go BB in my next tank)
2. If you have corals on the sandbed (brains, plates, clams. etc.) they may get up-ended by the crabs as they move around and burrow underneath them. There is also the possibility that they could burrow underrockwork and cause it to fall if it is not right on the bottom of the tank
3. I do feel they need to be fed directly and can't survive just on the critters in the sand especially with any other sand bed scavengers in your tank. They are not too bad to feed though as when I feed frozen mysis every other day or so I just make sure to get a basterfull of mysis underneath them.
4. I've read in the wild they get big, upwards of 20", I don't know enough about them to say if they would ever reach that in an aquarium and if it takes 2 or 10 years.
If you are interested in giving one a shot let me know, worst case scenario you could always boil it up and serve it with a little garlic butter if it does not work out
