The silica isn't an issue. Silica sand won't leach any usable silcates into the water, and even if it did, it wouldn't hurt a thing. The reason i was told not to use one, was because if the power goes out for a few hours undetected, and then comes back on, that it would possibly poisin the tank. The fluidized bed being completely void of oxygen, goes anoxic pretty quick with no flow, then when the power comes back on, it flushes all that toxin into the system.
It's been a while since i asked about one and was told about this, so i may be off a little, but it was something along those lines that i was told when i checked into them. The person i talked to told me they were fine for fish only, but very risky to use one on a reef.