Skill isn’t a factor in some Classic fights, its luck.
Get a ton of Shaman calls on Nef? Wipe.
Get unlucky with the Def reduc on Battleguard? Hey Crit happens.
You screw up ONCE and it can get you (and several other people) killed. During progression if even one player dies it can be a wipe. Which is why min/maxing your stats and racials is key to getting that little extra.
Hence why people like to bring Dwarf Priests for Fear Ward on boss fights where they’d rather not have a boss turn and swipe/breathe on the raid.