Shadowguard > the Roof = ~30% chance anyone in the roof when the final boss dies will get a drop.
Crimmy/parox' swampy, etc = ~10% chance to drop on any ML peerless, but some of them can decay while they're on the boss if it takes too long to kill... Crimson Cincture - however long it takes for a "half Apron" to decay on the ground is how long you have to kill the boss and loot it.
Luck doesn't effect "pack drops" from most sources. (Ilshenar minors, Tokuno Minors (disabled), Dungeon Blackthorn, Kotl City (disabled)) are the only ones I can think of off hand that are effected by Luck.
the "rare drops" like the Slither, Venom, medusa tile (any SA peerless drop) = ~5% luck is not effected, the item can only go to one person who has earned looting rights on said peerless.
I don't have a link, this is just based on my experience, most of which was farmed extremely extensively - Medusa & Shadowguard.