The basic gist of it is rage. Rage gives a +2 bonus towards melee attack rolls, damage, and will save throws. This thing is basically a real game changer. I usually play my fighters by enlarging them and hiding them behind Jaethal (Jaethal is my go to tank. Valerie is second go to tank.). However, with the high strength + power attack + rage + other bonuses. You can just charge in on an enemy and immediately one shot it. Its quite rare for most enemies survive a charge. Of course since its unfair you're probably going to get an unlucky crit to the face. Nonetheless this set up has allowed me to play far more aggressively than I could in the past. Its a breath of fresh air to be able to play like a monk, where I can just charge and poop on enemies.
Armored Hulk was chosen purely for Resilience of Steel, which helps mitigate the chances of getting a crit to the face. 3 levels into fighter was mainly for armor training and extra feats. I'm still figuring out rage powers, but am set on the beast totem (Mainly for natural armor bonuses and pounce). Suffice to say this build is quite fun, and might be able to compete with my strength based monks. I don't know how I ever missed rage.... Or rage powers.... fack this is what one thousand hours of playing monk does to your brain.
Increased damage output alone (and Rage doesn't do that much) doesn't mitigate damage to the extent that a Barb tank is as effective as simply not getting hit in the first place. Your Barb is going to have a very low AC ceiling and you are simply going to be resting all the time as a result of that. If you don't mind that, then of course there's nothing wrong with it. But that doesn't make it a super effective tank.
>Your Barb is going to have a very low AC ceiling
You'd think so on Unfair. Heck this might even be true. However, it isn't, even on Unfair. If you can hit that sweet spot between 60-70 AC on Unfair, then you're good. Compounded with a +8 belt, onslaught, and legendary proportions; then this is easily achievable. Consider Valerie being able to hit around 68 AC on level 16 if I remember right:
Even 50 AC is serviceable:
No amount of AC will prevent you from eating a crit of course. However that's not the point. You're not supposed to sit there and gather aggro then soak up damage on Unfair. You're supposed to set up for ways in which you can do the most damage upon enemies. For example, choke points, choke points are used on Unfair to funnel enemies through a narrow pathway in order to abuse the flanking bonuses. But that isn't the point. The point is that the best defense is a good offense on Unfair. If you can kill an enemy fast enough within two to three rounds, then you don't need to worry about taking damage.
And this is achieved by Power attack. Power Attack is by far the most incredible means of dealing big damage. Let's put this into perspective. Power attack allows you to gain a +2 bonus on all melee damage rolls, which is further increased by 50% if you're using a two handed weapon. Compound that with a high strength user, and you will be doing stupid amounts of damage. Throw in rage and other bonuses to the mix and it gets even more stupid. By they way, rage's potency gets better and better the more you level up barbarian. At level 13 the rage bonuses increases to +3. It doesn't really matter which Barbarian you chose, although admittedly pack master would be a better fit. On top of rage bonuses, there are also rage powers too. Still figuring those out, but I seem to like the idea of pounce and reckless stance. Too bad the other totems never made it into the game. Though this won't sway vivi dippers, vivi dippers going to vivi dip.
Anyways, the point that I am trying to get across is that the interaction between power attack and strength is amazing. It's why power attack + dragon style on monk is my favorite build. It's what facilitates non sword saint builds to do big dick damage.
>going to be resting all the time as a result of that
You only need to wait 50 seconds after the battle. Then you can rage again. Level 17 barbarian completely negates being fatigued after rage. There are also specific items that are tailored specifically to barbarians in game.