This is true of Defiance and Regeneration tanks. They get nickel and dimed by henchman damage because they have no flat damage mitigation.
Invulnerability is a different story. Sure, it provides less resistance than Defiance, but it also mitigates a flat amount of damage on top of it all. If you've got 15 guys dealing 200 damage a hit, that's 3000 damage. A defiance tank resisting 80% of that takes 600 damage. An invulnerability tank resisting only 60% of that, but with a flat damage reduction of 150 damage, is only taking 300.
Of course, it works the other way around, too. If there's just one guy, and he inflicts 3000 damage, the 80% defiance tank takes 600 damage. The 60%/150 Invuln tank takes 1140. Ouch. As a result, Defiance tanks are better at dealing with single large enemies that kick out tons of daamage in large quantities, while Invuln tanks are better at dealing with crowds of weak enemies.
Regeneration tanks are trickier. They have no damage resistance save what is provided by their gear, but they can potentially survive (via regeneration healing) anything that doesn't have massive spike damage. A regeneration tank, therefore, wants to limit the number of attackers he's dealing with, and spread their attacks out so that they can't all spike him at once. Make sure to block any attack that deals large spike damage, and just block often in general. Your self-healing generates aggro against anything that considers you a potential target.
Regen worked that way in City of Heroes, but it had a reputation for being a very strong defensive set because it was set to handle a high baseline level of damage. It's also useful to be Regen in that
every buff set benefits you greatly, whereas for, say, Dark Armor Scrappers, you don't get much use out of Sonic or Thermal Shields. It's a large part of why Willpower, Stone Armor, and Invulnerability were considered so good and so popular: they mixed defensive types. Will and Stone give you some resists, some regen, and some defense. Invuln gives you a bit of defense and a lot of resists. That means they all scale well with
any form of buffs or heals. They've got a strong enough baseline to handle a fair number of enemies on their own, but if you toss Fortitude on top of 'em? In some cases, that was about as good as popping your Tier 9, to be honest.
Haven't played with Invuln in CO, yet, didn't realize it specialized in group tanking. I guess my next tank will roll that way, then. I still greatly dislike that no CO Tanks have "complete" defenses: there's always
some sort of hole. I dunno, I just perceive an injustice where some Archetypes are called Tanks, but their tools for tanking are inherently incomplete. I guess it's also somewhat exacerbated by my attempts to tank for Alerts where other folks in damage roles have Taunts on their powers, and the team-up mechanic will often place a tank in a team with no hybrid or support characters. (And that some hybrid characters don't actually have team buffs or heals... or least any that they
use, although the PuG-maker can't be blamed for the latter.)
-shrug- I don't feel very tough as a tank, and the game doesn't make the reasons for that clear. That smacks of bad design.