Bill Zebub
“It’s probably Matt Mercer’s fault.”
Again it's backloaded.
Let's compare it with another class that does something similar. The Beast subclass. The beast subclass, of course, needs to be one handed but as such can use a shield. It doesn't have to go in with the extra attack; instead it can tank - but when it does go all out on damage it gets an extra attack on the turn it rages. Or when it tanks it has a reactive AC bonus on top of the shield. At T1 the claws barbarian takes until about round 4 for the frenzied barbarian to overtake. Which is longer than many fights go on - but the claws barbarian can do this every fight.
Or let's look at the Zealot. The first foe the barbarian hits each round takes an extra d6+ half level damage. Which is about half the damage of an extra attack with a two handed weapon. Round 1 the zealot will be ahead, and round 2 it will be about break even. Again it takes the third round of the fight before the berserker takes the lead (and that if e.g. great weapon master's bonus attack doesn't proc). T2+ this of course scales better than the bonus action attack and you've two rather than one chances of proccing it.
So yeah, by the time the fight is over the berserker may have done more damage than a high damage barbarian from another path. Once per day - and it will have been slower out of the starting blocks. And that's only if the berserker frenzies while the beast and the Zealot are doing this every rage.
Your defence seems to be that in the specific situation of a boss with no minions (that is going to be torn apart by the action economy). So it's an edge case.
This argument seems to be using the most powerful subclass and the most powerful feat as baseline.
Rogues get very little benefit from Defensive Duelist feat, but I don’t hear anybody crying about how Evasion is bad design, or that novices will take the feat (which reads as if it is designed for Rogues) without understanding the overlap.
And, no, it’s not only useful on boss fights with no minions. It’s most useful when fights are longer, and you’re not one-shorting nooks.
Maybe our table is different but we have 5-10 round combats nearly every session. We have 1-minute spell effects wear off on their own. I love Frenzy, and happily pay the exhaustion cost.