Only 2 problems I can see. One is if your cambands comes loose or you need to readjust/retighten your tank underwater. The weight on the loose cambands can either be lost or make the readjustment a bit harder.
I doubt that would ever happen. I've been doing it for 7 or 8 years. The hard lead weights just don't move that easily. Further, I can't see cam straps coming completely undone like a weight belt. As for the fat (8") tanks, I use them too. Longer straps is all.