I just did a bit of reading and the exhaustion after Berserker's Frenzy is a significant setback to balance out a VERY powerful ability that more than doubles your damage output at low levels, with other perks like resisting the asinine Shove spam with Str check Advantage.

D&D Berserkers get one level of exhaustion after every Frenzy:

Originally Posted by "Dungeons and Dragons"
Exhaustion Effects
Level Effect
1 Disadvantage on Ability Checks
2 Speed halved
3 Disadvantage on Attack rolls and Saving Throws
4 Hit point maximum halved
5 Speed reduced to 0
6 Death

So while still powerful, you have to consider when to use it and proper rest restrictions would keep the Frenzy spam in check.

Fighter is not only completely lackluster and boring but also mechanically WEAK after this take on Barbarian. 50% damage resistance, +2 damage and extra attack every turn, ok then. And I'm not a rules lawyer or a balance fanatic. I hate how Pillars of Eternity is balanced to death. But Larian doesn't seem to have a clue.