I have noticed that some enemies eventually become immune to stun/freeze effects.
Is there a breakdown of how this works?
From what ive observed, its seems like some form of diminished returns applied to each CC on a given target, where an enemies CC length is reduced each time it is CCd, eventually being so short, or being stunned/froze for so few frames, that its imperceptible to the human eye.
When monsters are hit by a “Hard” Crowd Control (see below), they build up Crowd Control Resistance (CCR) at a rate of 10% per second the crowd control lasts. On application, the duration of any hard crowd control is reduced by this CCR, which is capped at 95%. Therefore this means the duration of a crowd control can be reduced to the point where it only lasts 5% of its base duration if the monster has built up 95% Crowd Control Resistance.
Note that only subsequent Crowd Controls are impacted by this increased resistance (not the one that caused it). For instance, Mass Confusion will charm the monsters for 12 seconds (assuming they have no resistance from previous crowd control effects) and immediately applies 120% crowd control resistance on the monsters (reducing the duration of any subsequent Crowd Control by 95%) just after the application. This resistance starts to decay only after the Mass Confusion’s crowd control effect expires, after 12 seconds.
Mass Confusion in action
Even if Crowd Control Resistance can reduce the duration of Crowd Controls by up to 95%, it does not have any upper cap, eventhough the duration threshold limit (see below) acts as a soft cap. This lack of upper cap becomes obvious with Skull of Resonance + Inspiring Presence which applies a 30 seconds Charm, building up to 300% Crowd Control Resistance on monsters affected, preventing them to be pulled for 47 seconds after the Charm ends, until their CCR drops below 65%!
Indeed, Monsters lose the Crowd Control Resistance they have accumulated when they are not under the effect of any Hard Crowd Control effect, at the rate of 5% per second.
Yeah, the crowd control changes were implemented back in patch 1.0.5. Prior to that patch (iirc), enemies in high difficulties were simply either resistant (as in the crowd control barely lasted long), and/or almost unaffected by crowd control abilities.