TBh, I don't think M-Pinsir is *that* big an issue. First off, it has only one good option- it cant afford to buff knock off, or any other neat tools. Secondly, it isn't stupidly fast, so it can be outsped. The real reasons, however, are keys and 4 mss.
First off, M-Pinsir requires feint. Its the only reason it is good. It needs protect, too, or else it dies to mlop. This gives it 2 move to play with. No matter what it picks, it gets walls- Without eq it is blocked by meta/mmeta (it still is, but less so) & doublade, without close combat skarmory ignores it, without rock slide/koff it is walled by Zapdos.
Secondly, Its just a little too weak. With Banded Zor, Mmeta, and Azu all being easily able to live a hit (while being impossible to switch in on), Pinsir often finds itself sacking its teammates. MMeta is the worst of these, as it can switch in on any move bar eq/koff quite comfortably.
Finally, there are the keys. They can twave, magnet rise, ignore attacks, and drain with draining kiss. Pinsir is forced out by them, and they can easily provide their teammates with setup opportunities. It might still be ban worthy, but it isn't as OMG I'm broken plz bad as everyone is claiming.
On a similar note, Banded metagross is obscene. It hits over 600 attack, has a 100 bp, spammable stab priority, and can just rip open pinsir/Azu/what have you. Its perhaps the best revenge killer in the tier rn, easily able to take on most of the sets theorymonned thus far with ease.
Uhhhhh
If Mega Pinsir gets off an SD for any reason, lets look at some calcs of would be revenge killers. These are common mons put on teams to attempt to beat this. I think you aren't comprehending how ridiculously powerful feint is after a boost, and how easily this beats would-be checks.
+2 252+ Atk Aerilate Mega Pinsir Feint vs. 248 HP / 0 Def Zapdos: 311-366 (81.2 - 95.5%) -- guaranteed OHKO after Stealth Rock
+2 252+ Atk Aerilate Mega Pinsir Feint vs. 0 HP / 4 Def Thundurus: 361-426 (120.7 - 142.4%) -- guaranteed OHKO
+2 252+ Atk Aerilate Mega Pinsir Feint vs. 0 HP / 0 Def Heatran: 258-304 (79.8 - 94.1%) -- 43.8% chance to OHKO after Stealth Rock (scarftran)
+2 252+ Atk Aerilate Mega Pinsir Feint vs. 248 HP / 0 Def Mega Scizor: 406-478 (118.3 - 139.3%) -- guaranteed OHKO
+2 252+ Atk Aerilate Mega Pinsir Feint vs. 172 HP / 0 Def Azumarill: 654-769 (170.3 - 200.2%) -- guaranteed OHKO
+2 252+ Atk Aerilate Mega Pinsir Feint vs. 0 HP / 0 Def Metagross: 216-255 (71.7 - 84.7%) -- guaranteed 2HKO after Stealth Rock (if you take any chip damage, you just die)
+2 252+ Atk Aerilate Mega Pinsir Feint vs. 252 HP / 252+ Def Skarmory: 154-182 (46.1 - 54.4%) -- guaranteed 2HKO after Stealth Rock (Magnezone is so good this can just be rendered irrelevant)
+2 252+ Atk Aerilate Mega Pinsir Feint vs. 248 HP / 0 Def Klefki: 294-346 (92.7 - 109.1%) -- 87.5% chance to OHKO after Stealth Rock
+2 252+ Atk Aerilate Mega Pinsir Feint vs. 248 HP / 252+ Def Klefki: 207-244 (65.2 - 76.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
This thing's very few checks have to sacrifice almost all of their health in the process of just checking it. Furthermore, many common, viable resistances are just RAPED by its obscene power. Anything with +2 priority that is doing THIS to mew should not be legal:
+2 252+ Atk Aerilate Mega Pinsir Feint vs. 0 HP / 0 Def Mew: 543-640 (159.2 - 187.6%) -- guaranteed OHKO
For reference, that is over 50% stronger than Altarianite Dragonite's +1 Espeed in Mix-and-Mega
+1 252+ Atk Pixilate Dragonite Extreme Speed vs. 0 HP / 0 Def Mew: 348-411 (102 - 120.5%) -- guaranteed OHKO
It's stronger than AAA Adaptability +2 Bisharp's Sucker Punch
+2 252+ Atk Life Orb Adaptability Bisharp Sucker Punch vs. 0 HP / 0 Def Mew: 486-572 (142.5 - 167.7%) -- guaranteed OHKO
It's stronger than +2 Mega Lucario's Close Combat
+2 252 Atk Adaptability Mega Lucario Close Combat vs. 0 HP / 0 Def Mew: 542-638 (158.9 - 187%) -- guaranteed OHKO
When something has +2 priority that is stronger than the most powerful attack of an UBERS WALLBREAKER, you have a HUGE FUCKING PROBLEM. NO, this is NOT manageable and it's basically impossible to switch into period even running something like Skarmory. It's few reliable checks have to cripple themselves to beat it, and offensive teams literally get 6-0'd because it's so obscenely powerful. Get this shit out of our tier.