The question to ask with IBs is really whether the optimal Warband build really affords any of the extra tanky abilities?
Lets assume a 70+ IB w/o the extra 2 points from full Sov.
RoR.builders - Ironbreaker
If you bring Ancestor's Fury, Oathbound, Runic Shield, Punishing Knock, Earthshatter, and Told Ya So, then you cannot afford GnM or Shield Mastery and you have 1 tactic slot open assuming that you consider Toughness tactic to be mandatory.
You could use that tactic for Armor (Mind that you have pots and self-buff), the other tanky one you could use is Seasoned veteran.
The problem is that quite a few of these are "selfish" choices meaning that Ancestor's Fury, Runic Shield, earthshatter, and PK are all meta because they actually benefit the group's offense/damage mitigation in achieving kills.
TLDR: You can build a very tank tank IB that will be the last thing to die for sure but at what cost? My issue is that Borc, Chosen/Knight, and BG all bring the tanky stuff in combination with benefits to the grp more easily than the tradeoff IB is forced to make.
Edit: Come to think of it, given IB's lack of block buffs, the nerf to parrying guard damage probably hit them the hardest since high parry was the easiest thing to stack so really IB is probably over represented in these answers.
Brynnoth Goldenbeard (40/80) (IB) -- Rundin Fireheart (40/50) (RP) -- Ungrinn (40/40) (Engi)-- Bramm Bloodaxe (40/83) (Slayer) and a few Empire characters here or there, maybe even an elf.