They probably don't screw up most wishes, but we don't get episodes about "the wish that went perfectly well" because there's no conflict, and thus no story
Remember how Timmy wished to stop time and never age for FIFTY (50) YEARS so he wouldn't have to grow up and lose his faeries?
Nothing bad ever came of that, really. At the end he just apologized and un-stuck time, but he still got his extra 50 years of faeries to play with. No repercussions.