I had one a long time ago.
The main problem is that there are virtually no advantages to a fluidized bed filter. They don't achieve anything that a basket of bio-media doesn't do just as well. If you really need more filtration why not just get another canister?
You complicate the plumbing and put yourself at risk of blowing a cloud of sand into the tank or filter.
The textbook advantage of a fludized bed filter is efficient space usage, but the extra plumbing you need will offset that anyways.
I'd take a sump over a fluidized bed no question.