If you have sand, then first and foremost you risk slurping most of it out.
If you have a functional deep sand bed, you will disturb the zonation of the sand, destroy the microfauna that is critical to it working, and potentially release pockets of hydrogen sulfide which can wipe out your tank.
If you have crushed coral, you should IMO siphon the sand. If you have a shallow sand bed, I still wouldn't recommend it, mostly because you will kill off highly desirable critters.