90% of the good players I know always advise against putting points in IF... including myself.
Thats the problem, every skills should have a possible reasoning into spending points in mastering them.
Atm it is only a dump skill for when you dont have anywhere else to spend points
OR if you are going for a full strength character (and when I say full, I mean 30+ STR) in combination with a heavy or very heavy armor. Hit points doesnt mean jack shit if you are going naked, even my 101 hit points get rekted in 2 hits if I'm not wearing armor, but if I have an armor that makes me take only 5-10 damage per hit, then I have 3-5 extra life saving tanking.
But if you're wearing only medium armor, even with crazy amount of IF, it will only give you 0-2 extra hit before dying, thats just awful efficiency. For the other "normal" people out there, even with 7 IF you will barely see the difference, sometimes you will have a black bar instead of dying on the spot but thats it.
IF need a re-balance so that it grows from "almost always useless" to "kinda nice in some situations". Like every other skills.
Maybe make it +1 HP, +1 armour. Or make STR give no HP, increase base HP and make it about +3hp per IF
+1 HP +1 armor seems better IMO
Iron flesh = a flesh (the naked body) that have some kind of resistance and doesnt need piece of armors
But the buff needs to be smart enough so that crazy strength characters arent completely impervious to any damage. Like OP said, the ultimate maximum armor with +1 armor per IF would be near 90 armor. Would mauls and big-punch weapons dent such armor?