If you've got 1001 "gems", it's a fair bet that a few of them are going to be paste... :-/
The Fiddler's Fakebook has its share of duds too. Although mostly for me they're the old-time tunes, which seem to have survived by virtue of being so nursery-rhyme-simple that you could probably twang them on a fence wire.
O'Carolan's Concerto is on my "don't-like" list. It looks good on paper, but when you get into it, it's some unholy mix of sanitised-Irish and substandard-classical. It sounds like a parody/pastiche, but I can't work out which tradition he was parodying with it. Possibly both.