I don't think Mienfoo is broken. This is mostly based on my own experience, but I've never found it to be problematic. If anything, I think it's healthy for the metagame because it helps keeping threats like electrics, Shrew, and Growlithe-Hisui in check. I also think Shrew would become much more annoying if Mienfoo were banned.
I know this isn't the best way to judge whether a Pokémon is broken, but as I said before, I've never felt that Mienfoo was oppressive or frustrating to play against. I'll also share some stats later to support my opinion.
On the other hand, I honestly think Vullaby is much more concerning than Mienfoo, and if I had to suspect something, it would be Vullaby. Before LCPL we didn't really have enough high-level tournament data to argue that it might be broken, but now we do.
In every LCPL game I played, Vullaby was one of the biggest factors in making progress. Its versatility makes it incredibly difficult to deal with. It puts a huge strain on teambuilding because you almost always need specific answers to its different sets. You often need to dedicate a Tera just for Ground Vullaby (for example Tera Grass, which often becomes a 50/50), Tera Water Elekid, Tera Electric physical Vullaby in some matchups (also leading to 50/50s), Tera Water Gothita, Giga Drain Toed, or even Tera Water Pinch. More or less every team is forced to account for these possibilities, which I think is a much stronger sign of an unhealthy Pokémon than anything Mienfoo currently does.
w1 nplot tect slash hwave timid vulla
https://replay.pokemonshowdown.com/smogtours-gen9lc-940963
Turn 2 is a 100% kill since tinka was oran and vulla was timid
Turn 13 tect helps me checking smash or fblitz
Turn 15 again another speed tie
Turn 26 super huge hwave gives me the advantage, job done
w2 scarf adamant vulla
https://replay.pokemonshowdown.com/smogtours-gen9lc-943431
Turn 1 big kill, could sky avoid it? I don’t think so cuz you don’t know the set if it’s regular physical set you are pretty down if you put mudbray on knock, clicking with foo and putting mudbray in was def better if vulla wasn’t scarf, that’s why he stayed in and it makes sense.
Turn 21 his vulla goes crazy, I def choked cuz I had sub but here we can see how a set can change a battle even when you are ahead. After that I was down.
At the end my vulla clutched thanks to knock and I won.
w3 t ghost nplot vulla
https://replay.pokemonshowdown.com/smogtours-gen9lc-945893
Flinch broken mechanic.
w4 nplot t ground vulla
https://replay.pokemonshowdown.com/smogtours-gen9lc-948955
Turn 50 I was pretty down, vulla saved my ass and zs choked with vulla but I do believe vulla was super good after killing tinka.
w5 np slash hwave vulla
https://replay.pokemonshowdown.com/smogtours-gen9lc-950741
It was the worst set vs glimmet if I had t ground it was super annoying for his team.
Even that, it was crucial in the game.
Turn 48 is the key turn.
Turn 50 his vulla goes crazy with the flinch and max roll on turn 51 ending the game with a 50/50
w6 mixed vulla with tailwind
https://replay.pokemonshowdown.com/smogtours-gen9lc-952993
Same team used vs sky but different vulla set.
Turn 24 and 25 I completely choked, clicking tw would have given me the win.
Finals physical tera ground vulla
https://replay.pokemonshowdown.com/smogtours-gen9lc-961148?p2
It did nothing, his vulla won the game and we can see another nplot vulla winning games.
This is what I mean by true versatility in both its movesets and Tera options. As we can see throughout these games, Vullaby had a massive impact and, in some cases, even single-handedly won games.
Foo lcpl overall moves stats:
Swords Dance had a 47% win rate, while Substitute had an 86% win rate. As far as I remember, there was only one Substitute set that wasn't paired with Swords Dance; all the others ran both. Looking at the other moves, the results are fairly balanced. It seems like the real issue is specifically the Substitute + Swords Dance set, which can be extremely annoying to deal with, especially if your main answer is Foongus. If you're using Mareanie instead, the matchup is much more manageable.
Vulla lcpl overall moves stats:
Air Slash had a 61% win rate, Nasty Plot 74%, Tera Blast 67%, and Dark Pulse 50%. These numbers show just how threatening some of Vullaby's sets can be. That's why I strongly believe Vullaby should be suspected first.
Some people argue that banning Vullaby would make Mienfoo much more oppressive, and that could end up being true. However, the metagame and teambuilding would naturally adapt. Teams would no longer be forced to dedicate so many resources to covering Vullaby's different sets, and there would likely be more flexibility to use Tera defensively against foo.
If Vullaby is suspected and ends up being banned, and Mienfoo then proves to be too strong in the new metagame, we can always suspect Mienfoo afterward. For now, though, I believe Vullaby should be the priority and the stats confirm that