Pacification works on *some* bosses, and it's really a crapshoot like with most status effects. The safe rule is that, if Pacification would allow you to completely avoid a specific fight mechanic, you can't use it on that boss.
As to what it does, it's hard to notice the effects because it prevents the boss from using certain abilities, but, basically, if you use it on the boss at the proper time, they'll be incapable of using special attacks that aren't spells (Ifrit's cleave is an example of this). Bosses don't spam their special attacks like players do so it's really hard to notice, especially if you use it during a time when the boss wouldn't have used a special attack in the first place.
NPCs have a roughly 10% default chance to crit for 50% additional damage with their auto-attacks. They can't crit with their special attacks (mainly because a crit spike attack like Mountain Buster would just RNG gib the tank), and auto-attacks can make up a highly variable proportion of the boss's total DPS so it's hard to explicitly quantify how effective at reducing damage it is; auto-attack damage is always a minority of total damage dealt, however. The real value in it is in making incoming damage *predictable* and limiting the worst case: by removing crits from the equation, there is less variability in potential incoming damage and the worst case is limit to normal hits rather than crits.Bonus question, how effective is awareness is for bosses and how often/when do you rotate it in?
Of course, as it stands, Awareness is currently bugged such that it prevents any ability used on you to be a critical, which means that your healers can't crit heal you. Since the effects a healer uses on you that can score a critical hit make up nearly 100% of their output whereas the effects that a boss uses that can score a critical hit are only a small proportion of their total output, you pretty much never want to use Awareness since it will almost always do more harm than good (especially if your healer is a SCH; Adloq crits are *amazing*)