So first off, there is only Three forms of damage mitigation in this game where this discussion is concerned, damage reduction skills (Standard tank CDs), HP based shields, and damage reduction applied to the boss (Reprisal, Addle, Feint). Tank invulns exist, but they're whatever. Damage reduction, personally they are easier to balance, no need for "bells and ribbons".
HP based shields, are incredibly difficult to balance against standard damage reduction, as damage intake increases, their ability to mitigate lessens if the hp doesn't meet the same scale of increase. So on undergeared tanks is becomes a detriment while it's effect duration also ends a lot sooner than the standard damage reduction skills.
Damage reduction debuff, at 30% will be rediculously broken on raidbusters and more often than not pointless in dungeon content, unless it's AoE, and then making it even more broken in Savage and Ultimate fights with multiple enemies.
Adding secondary effects, unless they're all damage in some form like Vengeance, there will be complaints. "Why do they get more damage from theirs, while mine gives me more defensive than I don't actually need."
