Anything with Dark Miasma is great for heals/CC/buff/debuffs.
But the synergy with demons is absolutely disgusting.
3 holds in play between Tier 3, Miasma and the Dark Servant (I regularly hold AVs).
Can probably hit -100 res when focused on one target with Miasma's -30 for tarpatch, the 28-29ish from 3 whip strikes all 10 seconds, and the 2 ranged/1 cone AoE from each of the two ember demons doing -15 to -20 for 5 seconds each.
There are two ice demon attacks that DON'T slow and -recharge. The demonling's melee attack, and the aura (I think actually PBAoE attack) from the Tier 3. Combine with 2 slows, one with a brutal -recharge from Miasma.
An LT demon has a single-demon heal and a mass heal (you/allies included). Miasma has an amazing debuff/mass heal that the Dark Servant also casts.
Miasma's group stealth aura + an LT's ember shield give pretty wide res coverage to all but energy/dark-energy IIRC. Big psionic res on my MM and energy/dark-energy fills in from a ton of set bonuses. I'm tempted to slot res in to the support demons for a bigger ember shield boost but it's kind of a bummer since only one of them provides res.
What Miasma provides that that the demons don't: Lots of -ToHit and -dam, a small spammable -regen (from the same debuff/heal) and a huge -regen on a power that also stuns, fears the target, area slows and -recharges 50%, and happens to resurrect any KO'd allies nearby... AND does a smidge of AoE dark energy, rounding out the demons already wide damage-type range of lethal, fire, ice, and toxic and giving that power what is probably the longest list of eligible sets in the game. Oh and don't forget that totally awesome cone of fear with the -ToHit
To top off all that CC/debuff, demons have a sleep.
And the AoE is better than a lot of people think.
It's not just two demons with breath attacks. The three little ones get those as well. They all have a 30 degree arc and are 40-50 ft IIRC. That's a lot of spray. The Tier 3 gets a huge 130 degree arc attack that goes 60 feet. There are like 2 10-15 radius ranged AoE attacks. There's a persistent toxic aura. And I think there's two PBAoEs. Your biggest whip attack is AoE.