UGF's were once popular and they actually did a of a job. A UGF requires substrate cleaning every week. That said
They are no good for any fish that digs the substrate as they often go down to the UGF and stop it from working. They are also no good for the planted aquarium which are far more popular now than they were ten years ago. Perhaps thats the reason for decline of the UGF. If you have a tank with substrate and movable rocks,
driftwood only with non digging fish, then a UGF is cheap and works very well. A UGF works very well with Haplochromines for example.