Well ancients don't have cursed or conviction. In normal PvM, anything that doesn't have amp damage is never going to kill you or your demon anyway, and the %dr on your demons can become very high or fully physical immune, but amp damage is glitched right now and gives -100% DR to enemies and summons that are immune and -200% dr if you have bone break vs immunes (including bound demon amp damage, hence prebuffing with bone break helps). But this also means even a 116% dr stone skin venom lord will have 16% dr after amp. If you're using an urdar from act 3, it has 55% base dr so 71% dr after engorge/blood oath, and immune to fire/cold/light. A lancer would have 49% dr and immune to fire/cold/light. Of course, all this begs the question of why you'd be doing uber ancients in the first place with a taintlock instead of a smiter or mirrored blades or whatever setup, because its pretty much one of the few niche things in the game where tainted aren't the best build.
Those megademons are only the ones in furnace/ubers, so its not a very 'free' way to do anything unfortunately :S you can get +4 all skills and holy fire / %dr from hephasto consuming too. But its quite impractical, all it lets you do is get the highest possible tainted damage prebuffing if you were on a 3x tainted build with no bound demon, but you need 20 points bind demon to get it. Then you could bind hephasto, get a skill shrine, and get up to level 63 tainted on a mang song setup. So unless you're being a sneaky bastard in a dueling game its not too useful, since nobody would put 20 bind demon and then not use a bound demon.
The wiki is still using 3.1 numbers afaik, its definitely +4 skills on hephasto now and +0 in 3.1 (since it was glitched not to work, but fixed and buffed in 3.2)
Well my understanding is that the colossal ancients' respective aura reduces enemy resist by 100%, so my demon's resists probably should have been pierced... but I didn't actually check either. Their hp didn't seem to budge, though. The pin head was an absolute tank. Also, that's too bad that megademon6 is from the furnace, but kind of cool.
I did uber ancients on him because I realized just how high damage the bind demon was striking for (easy to see with how quickly Baal dies), and because you can load into a game with max +skill gear and then swap to ANY gear/charms you want without a drop in the demons' damage. I got mirrored blades up to 5 hits with melee+sorb gear/charms on and it performed quite nicely with the attack speed boost from Purge + some leach. The tainted fireballs were a cherry on top.
It really suits long public tzone shard games to enter with the skill gear, you can easily just teleport through the game. Plus, you can choose to stash all your skill charms and run a game with nearly a full inventory's worth of space. Maybe this was already covered in 40+ pages of thread

but the freedom is the main appeal beyond its performance IMO. IDK about the AI attack behaviors, but I wouldn't mind a definitive guide on how minions stack + how external factors change things like monster/demon AI(join order, etc). It's also interesting trying to balance the best cube buff for rapid-joining games like when hosting WSK tzone runs.
One thing I've noticed is that the floating chains animation on a bound demon "reset" when other players teleport near me. Pretty weird. So if I watch a duel in A1 while standing still in my stack, every time they teleport the chains flash