I don't like the tier right now, Mosa is a big part of it, and Its not really because of any one factor as much as its just oppressively awful to account for in every aspect of play, unless you bulk up with something like Pex+Clef, as people before me have said, which believe it or not, makes for a pretty boring or frustrating experience.
Clever title relating to bugs or something
I'll go over the builder aspect first and then the in battle aspect second because they both represent different problems I have with the roach.
Building around the Roach
This thing lacks good blanket checks. A lot of "switchins" are very set dependent at best or are
Tapu Fini inconsistent U-turn bait at worst, the only switchins that genuinely cover multiple sets at once admirably without being worn down too quickly with U-turn+Hazards are Toxic/Pjab Pex and slightly Spdef invested Moltres. CB absolutely trucks Clefable, Tapu Fini, Aegislash, and Galarking depending on the moveset, and QD has the potential to push through even the better switchins, because they're all 20-30% at best away from being KO'd at +1. This by itself isn't really a big deal, until you compound that onto the fact that Mosa is an extremely difficult mon to offensively 1v1, so you're pretty much restricted to priority for taking it out, with the relatively awkward to fit and rare Azumarill being the only Priority user that actually switches into it, though Scarf Tapu fini kind of works as a backup check. Also worth mentioning that HDB can circumvent Priority to an extent but HDB also lacks a lot of immediate power so it's not really that big of a concern frankly, not to mention Rocky Helmet chip putting it in range anyways.
So when taking this all into account, it's ridicolous to Handle Mosa without just resorting to multiple defensive checks, because what little offensive counterplay that does exist banks entirely on something else getting them in via pivots or coming in on a KO, making them rather inconsistent, and what defensive counterplay exists usually cannot reliably handle both CB and QD, and if they can they tend to be very close to being overwhelmed, pretty much mandating a backup check regardless. This is frankly a ridiculous amount of care required to not lose to one Pokémon unless you're already running some generic Fat Balance, and I didn't even really touch on how the natures used can wildly change its offensive counterplay due to Beast Boost, though this applies to CB much more than QD.
Actually playing around the Roach
This Pokémon is ridiculous in battle, flat out. Going over CB first, the amount of support required for Mosa to push past most of its counterplay is rather minimal, especially when you consider that you don't have to beat just Pheromosa, because this mons a U-turn Bot. This forces you to both handle it and whatever random breaker it decides to bring in, making the amount of pressure it can apply to a team immense, since it poses such a huge threat by itself already. The consistency with this set VS. Offensive builds specifically is frankly unparalleled, it hits SO hard and forces switches so easily, on top of having a pretty solid amount of unpredictability with its Coverage options, between Throat Chop, Drill Run, Ice coverage, and Pjab. Like I mentioned previously, another unpredictable aspect of this set is that the nature (which you have no way of telling which is which until it attacks) determines how you go about taking it out to an extent, with teams that rely on measures like scarf Fini/Lele/Etc. largely finding themselves very overwhelmed when they're staring down a speed Beast Boost Mosa randomly.
We aren't done yet tho, because Mosa also has the potential to be QD, and no it is not as obvious to determine as people make it out to be. This set does indeed fit on Balance-y builds admirably, and can completely catch you off guard and annihilate you with some decent play. Most of the time, stopping this set from getting a QD off is a 50/50 and for more offensive builds, Tapu Fini takes any amount of chip and then they have to pray you miss Focus on their Steel, which is very engaging and not at all extremely frustrating, as one would imagine. The worst part is that Teleport has made bringing in shit like this easier than ever, so relying on just not letting these sort of frail, offensive mons in is practically impossible nowadays. This things just very easy to support for how devastating it can be in an instant, on top of being relatively difficult to predict, albeit far from impossible.
IDK lol
It's dumb, not a lot to add that other people better than me haven't already said 100x better. Whenever I decide to finish reqs this weekend, I'll definitely be voting Ban, this mon's offensive MU is ridiculous and it forces pretty derivative structures onto fat teams whilst enabling much of the tier as a fantastic fast pivot.
Oh yea and I forgot to mention Physdef Volc is technically the best all around counter to this thing but it has a lot of issues atm and generally doesn't slot cleanly onto teams without making concessions vs stuff like Urshi.
Edit bc i dunno lol: losing my 5th alt to an ada lando roll has made me realize reqs are grating and unfun so i take back saying i'll vote for my sanity :p
