Spoilericity definitely hampered by a severe lack of context in almost all cases. This Troper has watched all the (safe for work) AMV Hells multiple times before watching some of the series (Cowboy Bebop comes to mind) without any major effect on the viewing experience. The only real exceptions are the pretty blatant deaths.
Squick - Pretty much the entire point of AMV Hell 0 and AMV Hell Divided By 0. There's a reason those two come with a huge content disclaimer.
The Weird Al Effect - for everyone who goes about watching an anime series and goes "hey, I recognize that footage from AMV Hell."
Or hears a song somewhere and goes "Hey, I recognize that song from AMV Hell" Even worse when it makes you remember footage from 0 or /0.