I’ve never seen anyone come into these forums and say
“I played as a frenzy barbarian and it sucked!”.
We discussed this earlier, so that's a bit of a non-point you're making. I guess you didn't read all the posts though, who does?
Berserker is overall a solid Barbarian subclass. The L3 is hard to use well but powerful, and some DMs (clearly including iserith's DM) let you get away with basically no-consequences for the Exhaustion (5MWD will do that, and so will letting the player not roll stuff, as will doing stuff like letting people just take multiple long rests in a row to ditch the Exhaustion). L6 is very solid, a great ability. L10 is complete trash that's basically a joke (rare that I say that about an ability but wow. It's not just "niche" it's basically unusable unless you have a high CHA, which likely means rolled stats). L14 is extremely good to the point of bordering on the overpowered.
It's a design that they simply wouldn't do, now, in 2020. The L3 and L10 stuff (and likely the L14) would have been implemented differently. No way a designer now would go with CHA for the L10 for example, and the L3 would likely be weaker but more usable.
But none of the other Barbarian subclasses wildly outshine it, because they all have their own issues. Even if you literally never used the L3 or L10, it would be about as good as some subclasses (especially some Totem Warrior variants). The Zealot is probably overall better in most RAW-oriented groups, I'd suspect (opinion, not fact), but it's narrow and situational, because if you often face no
real consequences for Exhaustion as iserith repeatedly describes, then Frenzy is amazing.
And the Barbarian class chassis is great, I think. It's a really nice class, even without a subclass. Definitely my favourite implementation of Barbarian in any edition.
People seem to assume that every battle requires frenzy or rage. They do not. I’ve gone while adventures without using frenzy once.
That's exactly what I'd expect, and an adventure where you don't have it means it might as well have not existed in many ways (not all ways). Whereas iserith seems to be saying he can use it basically all the time and just chat the DM ("player skill" as he puts it) into letting him not roll for stuff (sounds like he has a massively soft-touch DM to me, esp. as his Zealot has also never been killed, but hey whatever is fun!).
Anyway, if more Barbarian subclasses were competitive it'd be more criticised, but because they aren't...