This also contributes to the game being too easy. Obviously bosses have saves for a reason, if they have well designed saves, and also select RESISTANCE then blanket immunity just makes them one dimensional surround and pound encounters.
On my CC and tactical builds getting to the boss is the worst part of every raid and quest... straight DPS is boring as ****.
On the other hand imagine a Raid boss:
Three players land stuns (boss fails his hefty save) at roughly the same time = 3 second stun immune to further stunns for however long works to maintain balance (maybe 10 secs?)
Or two players land Trip (Boss fails his save) falls down for 2 seconds and can't cast/hit or land specials immune to further trips for a few seconds.
Or two casters land Hold Monster (fails very high spell penn and fails save) which of course has a much reduced effect on a Boss, say 1 second for every caster that lands, boss is held for 2 seconds. with same short term immunity after.
Not only would stuff like this lead to much more dynamic boss battles, it would increase the value of CC and tactics. But that's not all, it would allow bosses to have more formidable special attacks. Imagine a Legendary LOB but his Stun attack stuns players for 1 minute no save, and his cleave (whirlwind) does one shot 3k damage regardless of PRR say this whirlwind lasts for 3-4 seconds from the start of the "tell" animation, everyone calls out "whirlwind" and the Tank and two off tanks hit trip within ~1 second of each other, LOB falls down and his one shotty whirlwind doesn't kill half the raid. If only 2 land trip he still gets 1 second of whirlwind and kills a couple people potentially, if only the tank lands trip he's only lost 1 out of 3 seconds but this still saves some people potentially. Of course Slowing and otherwise impeding the boss could easily be a part of battles as well, a Slowed LOB hits less people with Whirlwind or his stun.
Ofc you can also require debuffing before anything works, and the debuffs can be limited for balance reasons, short duration, constant reapplication, or complicated combinations would all add dynamics to boss battles.Obviously they have to avoid allowing stuff to be too good:
(ss used without asking will remove if he objects.)