You can only have as much biological filtration as the fish load supports. Fluidized bed filters work and work well but are more complicated than most people want. They also suffer from the problem of very rapid die out in the case of a power or pump failure. (This is true for ALL completely wet bio systems.)
I've used FBF systems before but wouldn't pick one over more conventional methouds. With a sponge filter...bio-wheel...ceramic rings ECT. you can transfur some of the established media as seed material for new aquariums. Thats difficult if not impossible to do with a FBF system.