Oh, maybe they're not all bad. But some of them are just terrible.
A lot of times the band is made of metal so soft it deforms around the head of the little bolt holding it, and/or around the nut, making it impossible to tighten them. Sometimes the band is a hair too long, or the metal has stretched, so the thing can't be made tight enough. You go over a bump and the thing shifts a little to one side, then the crank arm comes around and hits it... it can get ugly real fast.

Getting the length of the band just right is not rocket science, but it assumes you know the exact dimensions of the mounting tabs of the bottle cage. Get that wrong, and you have problems. If your bottle cage came with the mounting bands, it might be okay. But buy them separately, well... good luck with that!