Here’s one example but far from the only one:
Serpentstalker’s Trickery - Classes / Hunter - World of Warcraft Forums (blizzard.com)
My wife’s Ret pally got her BiS from the great vault quest, a dirt-simple guaranteed BiS.
Meanwhile, MM hunters are forming hunter-only groups to cheese their way through Plaguefall to kill Dominia every 5 minutes on repeat in order to get theirs to drop. People in that thread have run Plaguefall 50+ times with no drop.
Could these just be made a guaranteed drop, please? RNG adds nothing to this experience, especially when creating a different legendary is a massive opportunity cost.