A few potential issues, if you're putting stuff behind:
1) Using remotes. If you're lucky, you might get enough bounce back from the screen or the ceiling, but you might find you need an IR repeater (in which case the problem then (especially with several devices) becomes 'can you position the emitter somewhere suitable to cover all of them').
2) Seeing equipment displays. Not a problem for things with on screen displays, but awkward for things that don't if you're cycling through options or setting volume with nothing playing.
3) Long video cables if you're using a TV (or, as a positive, shorter ones for a projector).
4) Access. you still need to get to the kit to change discs etc, so with a low rack you need a gap to get at it (and a line of sight within the kits acceptance angle from the remote control or repeater emitter for IR remote controls to work).