MHera is very good but it does have quite a few flaws that stop it from being S. The biggest one IMO is that it loses to every other relevant wallbreaker in the tier 1v1. Lando-I has Psychic, MMedi can Zen Headbutt, MGarde Hyper Voices it, YZard roasts it, and it takes up to 333% from MPinsir's Return. I know that 1v1 scenarios aren't very relevant, but it's pretty nice to have your wallbreaker be able to revenge kill the opposing team's wallbreaker, therefore enabling it to come in against offense once something has been KO'd. Also, while its low speed isn't really a detriment vs offense (everything on there outspeeds the other wallbreakers too anyway), it actually can a problem vs stall and balance. No defensive Pokemon cannot afford to outspeed the wallbreakers that lie at base 100 Speed, but they can do so for MHera. In fact, Jolly is the preferred nature on MHera mainly for that reason, and some defensive Pokemon even creep that (Mew and SpD XZard, for instance). On the other hand, MMedi, MGarde, and YZard can easily afford an Adamant/Modest nature, which makes them even harder to switch into.
If there's a wallbreaker which deserves S rank, it's MPinsir IMO. I think it's about time we move this terrifying beast up, since it has been dominating the metagame for quite a while now. It is just ridiculously potent against defensive teams, and SD+Quick Attack lets it clean offense too. Its decent natural bulk means it can switch in on the vast majority of defensive Pokemon, and it also has an easy time switching into Ground and Fighting moves. Its SR weakeness is a problem, but it doesn't need to switch in too often, and even a Pinsir at 25% can put in a ton of work. Furthermore, what team doesn't have Defog or Spin support nowadays? MPinsir currently has no truly good defensive answers, since it is very good at wearing said defensive answers down, and has even developed a way with, when paired with Magnezone, dispose of Shed Shell Skarmory very reliably.
I wan't really confident before, but at this point in the metagame, I feel like we should move Mega-Pinsir to S rank
So we refuse lando-t for S rank and yet we consider the most one dimensional mega that might as well require the most support of them all for S, bar char-y (Honestly i'd even argue that seeing as you need hazard control
and a zone for pinsir these days, which is more support than char-y requires b/c scarftar is a good mon to start with).
Well where do we start. With the rise of shed shell skarm and doublade and rhyperior, its not as much of an issue as it seems to defensive teams. You could argue that "ooh they run knock off watch your ass out" but you could just as easily just run something like rhyperior or even
zapdos mega aero and check pinsir just fine. Not to mention that a knock off pinsir performs FAR worse against offense. It runs one set and one set only, that is SD with close combat or knock off. Its a severe flaw that the moment the opponent sees a pinsir on your team, they know the exact ev spread, moves, item (:P) and nature of it off the bat.
It does have more natural bulk than i initially expected, i WILL admit, but its really not that bulky when you have a crippling SR weakness and downright pitiful bulk pre-evo. I find the argument that you can wear down your defensive counters extremely flawed because that's assuming you can actually come in multiple times. This argument only works with mons like keldeo which have actual natural bulk and solid typing to actually wear down its counters with repeated scalds.
So sure, lets pretend that it does well against defensive teams with knock off, even though rhyperior still stands as a ridiculously solid counter. Lets then move onto offense. Double intimidate cores have skyrocketed in popularity and quite frankly, whenever im' using this core and i see a pinsir on the opposing side I really dont' have to give a two shits about it. Scarf Lando-T and Mega Manectric are both extremely solid revenge killers commonly found on offensive teams, and their presence really hinders the performance of any pinsir team considering they're always around late game when played smart. As if that wasn't enough, you have to deal with excadrill, scarf tyranitar, scarf heatran, scarf garchomp, scarf terrakion, scarf kyurem-b, scarf magnezone, thundurus, and talonflame, all of which reliably revenge kill you. That's way too many revenge killers for an S rank mon lol. Considering the stable popularity of sand offense and the rising popularity of double intimidate cores, i'd reason that the metagame is actually quite hostile to Pinsir rn and should stay down. It may be reasonable with knock off against stall and stuff, but its raped by offense easily.
Keep in mind that you require hazard control
AND a magnezone on the team
AND knock off on your pinsir
just to perform well against stall (lacking rhyperior!!!) That's far more support than even char-y, and atleast that has the raw bulk to actually check stuff. What does pinsir check, Conk?
Finally, I feel like people overhype the shit out of quick attack on pinsir. They feel like once you grab a boost you're just golden against offense or something. I'm not even talking about scarf mons or resists at this point, i can check a +2 pinsir with my full health latios after rocks easily, and that's kinda disappointing for a mon that relies so hard on its priority to "clean offense" and not even reliably kill a mon with 80/80 physical bulk after rocks.
TL; DR mpinsir is 2ez to revenge, not versatile in the slightest, needs considerable support and performs pretty badly against the current meta offense. Keep at A+