[ruiny]

I say shallow sand bed. There are some nice bare bottoms, but a reef needs sand if you ask me.
It isn't without it's challenges though, mostly keeping it clean and on the bottom on the tank. But it will also allow for some very interesting fish, like jawfish, scotters, some blennies, also some wrasses and many inverts like sand stars, naus snails and cucs.

I think a bare bottom is only for ultra high flow all sps tanks that are going to the sterile look. (which can be nice)