Debris will breakdown into nitrates, but it takes longer. Yes, if you leave a HOB or canister unmaintained they will do the same thing. So if you leave debris in there it will eventually cause a spike in nitrates (UGF, HOB, canister, wet/dry w/ bioballs (something reef keepers are usually well aware of), etc.). The normal nitrogen cycle is not what is question, that is always the same. It is the potential for debris buildup, which will cause a lot of extra nitrates, that is in question. UGFs have more potential for this than other filters. And if people are undermaintaining other filters, why suggest one that requires even more diligent maintenance?
Other filters are easier to counteract past actions of undermaintance. As in, if my UGF tank has nitrate problems from debris buildup, just like my HOB or canister filtered tank, it will be easier to remove the excess debris from the other fitler types and counteract the problem, as opposed to the UGF.