Part of the problem is the fact that the flow can often cause particulate matter to be gathered under the gravel. If people clean the UGFs every so often, that would be less of a problem, but it is a pain to do so (you must remove the gravel and then the filter), and larger fish can render these obsolete quickly because of their large volume of waste.
The success of the UGF relies upon particles being forced through gravel, so any uncovered gravel causes a pool of debris to form, which is not exactly the point of the filtration.
As Thom said, it is outdated, and there are some dire consequences if it is not properly maintained.