Annhilape was balanced and I will die on this hill. It has 4 prominent weakness of Flying, ghost, psychic, fairy almost every team carries atleast one of these, working around ragefist is super easy unboosted it's just 50 base power so cripple it with wisp/encore or boost your own Atk/Spa for guaranteed ko. The other attack it usually carries is drain punch which unless your name is Lucario you're not dying from it so setup is free.
in OU,
Flying types: Corviknight, Dragonite, Talonflame, Gyarados, Skarmory.
Of these Gyara is mediocre and lacks flying stab, Talon is passive and not very good, and Dragonite doesn’t really like running its flying stab because its a mid move and has better choices, while Corv and Skarm just get farmed by Taunt+BU.
Ghost types: AbsolZ, Ceruledge and Gholdengo.
Absol is fragile AF and dies +1 Rage Fist if it got boosted to 100bp by any prior attack (incredibly easy to achieve/force). And Gholdengo can’t even OHKO without specs and trades itself for Ape, who on some sets can just rest after the scuffle and wake up with chests berry. And Ceru is limited to certain offense teams.
Psychic types: Hoopa-U, Latios, Mega Meowstic, Slowking G, Slowbro, Mega Starmie.
Hoopa is hard to fit though it does work, Mega Meow fits on faster teams and so it doesn’t solve the problem Balance has against Ape, and both Slowbro and Glowking just lose to it. Mega Starmie needs Zen Headbutt to beat Ape. And remember none of these can even switch directly into it, but you also can’t just click pivot moves on it or you make Rage Fist stronger.
Fairy Types: Clefable, Magearna, Slurpuff
Clef straight loses to BU+Ape, not doing enough damage damage and especially against RestChesto just feeds it boosts. Mag is probably the best so called check, but there were sightings of Ape making its way through Mag anyways and still doing a ton of damage to those teams. And Slurpuff is of course a lead and HO only.
So when you break it down, it LOOKS like there’s many options but in practice many of them have shaky match ups or just don’t actually work against it reliably (can’t switch in which gives it time to boost with BU, often will survive a hit anyways and KO back) and several of these just don’t fit on balance and that’s the team which suffered the worst against it to an unhealthy degree.
There is three encore users in OU and its a weird fit on all of them since you have to give up a crucial slot. But Mag is the best user of this, and the surprise only works once. It’s a temporary stop at most. As for Wisp, only two Pokémon are faster and have room to fit it, Talonflame and Darkrai. The former again is not nearly as good anymore as past metagame, and the latter is a single Pokémon that dies to +1 Drain Punch if Wisp misses. Plus Darkrai can’t sufficiently threaten Ape who can just boost over and over to offset to burn and KO Darkrai anyways.
As for boosting, what Pokémon is boosting in front of it?? What balance team Pokémon has the luxury of being able to do this.
It's physical bulk isn't great so any super effective physical atk is killing it or 2hko if it has bulk up a few times.
Despite it's 90 speed it's slower since no one invests speed on Annhilape unless it's final gambit ape so you're always outspeeding it unless it's a definitive slow mon like Melmetal or Clefable.
It can get 3 bulk ups in a row and still get it's shit rocked in 100 ways while all Magerna needs is 1 shift gear for most games.
And this is just wrong. Especially vs balance and bulkier teams, even a single BU defense boost makes a huge difference and makes it very hard to get off the field without strong SE hits. Which as I pointed out above, isn’t exactly automatically fit on teams because the options available are not all reliable. Of the ones I listed that fit well on balance and can win against it decently often, there’s Gholdengo, Mega Starmie (maybe), Latios and Magearna. Talonflame is mediocre and not something you fit usually, and straight up loses without Brave Bird because Ape can just boost to offset Wisp (restchesto sets can even do this and then heal off the burn later). Yes, offense and generally faster teams have an easier time against it since it won’t get as many chances to boost, but the stuff it does to balance and bulkier teams and restricts them is way unbalanced and unhealthy.
Also towards the end of its time I know some players were using pivot spam teams with wish support to further support it and make it ridiculous.