While I agree that the demon commander is probably the hardest, I have beaten him several times on threatening. It really helps to have a keeper(or another barrier user), as it can completely negate his burst. Also, if you stay mobile, you can avoid his "pop-up" attack entirely.
If you get his rythm down, I would even say, he is the easiest of the bosses with a coordinated crew.
To me, what makes fighting demons the hardest is the combination of the boss(potentially lethal if uncoordinated) and the adds(green spirits with "homing" ranged attacks, Fear and Despair demons which add a lot of CC and burst, and Terrors, for that extra pop-up damage and AoE fear).
The Venatori is the easiest in my opinion. She doesnt hurt a whole lot and most of her abilities, while powerful, are easily avoided.
IMO, the Red Templars is a close second to fighting the demons. Stalkers can catch people unaware(and kill them), they have powerful archers and mages(that lirium spit) and the Behemoth packs a solid punch and mess up the fighting area.
The boss, while slow, have very high damage and lots of AoE. When he starts spinning, he is zoning atleast 1 guy(tip: hide behind a pillar or other terrain), his jumps hits so hard and his "splitting earth" line attack can hit multiple targets as well.
Throwing in some archers, a stalker and a behemoth, and you have your work cut out for you.
Anyway, when fighting bosses, it is good to have an assassin or Alchemist to backstab/shred the enemy while the are walking towards a different target(a wounded reaver works too).
TL:DR
All in all, I think the balance is about right(the Venetori is slightly weaker, but that is fine I guess). What I would really love to see, is more stability(crashes, missing skills, sudden DC's, failing to load into the match etc.), an option to mute yourself and a chat option.