Yet, at the same time, I wonder how many fics might be labeled UST but might not really be filled with UST because either the author felt pressured to use the label or might have a different definition of what UST is.

And? But? So? ;)

Seriously, not trying to be snarky, but...the only way to find that out is to start reading fic labeled (or mislabeled) UST, to find out what it really contains. And, well, I think it's a fair assumption that if an author put it as a warning/category then their intention to demonstrate at least one UST relationship. And if that wasn't their intention, well...too bad for them for buckling under pressure, because I'm not interested in certain USTs and they've lost me as a reader without me even looking at their story. ;)