thought it would be good to compare all of the subclass abilities side by side, to see how they both stack up.
Frenzy vs Divine Fury (Winner: Zealot): As noted, divine fury is about a match with frenzy. But if frenzy still costs exhaustion it loses ground.
Warrior of the Gods: Mostly a nice flavor add for the zealot, but I don't consider it worth talking about in true balance discussion.
Fanatical Focus vs Mindless Rage (Winner: Berserker): Fanatical focus is good, I really like it. But the true immunity of mindless rage is awesome, and its immunity to 2 key conditions that stop barbarians from being barbs, or worse can turn them against their own party. I feel its the signature ability of the berserker right now.
Zealous Presence vs Intimidating Presence (Winner: Zealot): This one is not even close. Intimidating Presence is not a great ability, you are sacrificing a lot of barbarian damage to use it, and its effect is too limited to be worth it in most cases. Meanwhile the bonus action of the zealous prescence makes it highly usable, and its also very powerful.
Retaliation vs Rage beyond Death (Winner: Berserker): Rage beyond death is cool as heck don't get me wrong. But I find the normal barb relentless rage already takes care of most of that. Its just really damn hard to put a barbarian down, by the time you get through all of the hitpoints, and you have hit the barb enough times that the con save of relentless rage becomes failable....the fight is normally over. Meanwhile, retaliation gets use over and over and over again.
so the subclasses aren't too imbalanced with each other overall. However, if you account for the fact that most characters never get to the higher levels, and I think Zealot comes ahead a bit. Honestly the frenzy issue to me is not the main problem. Its actually the intimidating presence, its a really weak ability overall.