Yes, one of the big problems we have in trying to explain undergravel filters (both UGFs and RUGFs) is that in the hands of a pro, there's nothing really wrong with their ability to operate and they are quite cheap. However, in the hands of the typical beginner they are a disaster in my opinion. Its just much easier to learn on and operate a good modern internal, HOB or external cannister filter for a beginner. (Not to mention that by the time you're a pro you might not want to operate a UGF anyway except by the circumstance of having already had the filter and having a lot of tanks and therefor needing it in a pinch or something...)
As Robby said, its design is mostly optimized for biological filtration, providing a large volume of gravel for bacterial surface use (but not necessarily the best type of surface although not bad.) There's no provision for temporary additions of chemical filtration, such as carbon, unless there are attachments on the pumping pipes. As a mechanical filter, the UGF is just kind of a mess in that maintenance is so difficult. The stuff all collects under the plates and in the gravel and gets all over the tank during cleaning attempts. In a normal gravel tank of course you see debris during gravel cleaning but with a UGF its more like if you took a cannister filter and dumped all the collected debris straight back into your tank. The trick is to link the siphon hose up tightly with the plate and hope to suck all the debris straight out but it usually doesn't work that nicely.
Then there's the problem that if you give in and get rooted plants (even though your're not supposed to with a UGF) then the roots will clog the underplate area so that the debris resists being sucked out during cleaning. Another problem is that the gravel needs to be pretty level and flat and of the right depth (since its serving as a filter medium) and that often works against your aquascaping plans. I don't mean to "pile on" and I know the plates can technically work well but I just can't help but be another of those who feel this is just "old technology" and most any beginner can do better than this.
~~waterdrop~~
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