Well, the only disadvantage that immediately jumps to mind regarding a floating platform is the possibility the vegetation may become too heavy at a certain point and sink the platform... a mounted or secured platform would avoid this problem.
Of course, you mentioned that this is a "custom" DWC system, so what you're building and what I'm visualizing (and I'm pretty fuckin' tore up right now, I've downed a few bowls of OG Kush in the past few hours) might be two completely different things. Having said that, I also thought that depending on what you mean by "secured", a secured platform may hinder your ability to add/change nutes, check water temp/ph/levels, etc. In order to recieve valuable advice it might be helpful if you post pics, or at least a good description, of the system you are building.