In all honesty, I believe that Pinsir-Mega is a broken Pokemon within the Monotype realm. Pinsir-Mega can set up Swords Dance on many things and most types don't have a Pokemon available that is well equipped to deal with it. Then it can proceed to sweep with it's choice of Return, Quick Attack, Earthquake, Stone Edge, and Close Combat. That's all I'll say for now because I want to hear all of your opinions on the matter. ^_^
I'm quite sure plenty of types can deal with Mega Pinsir. All you have to do is not let it set set up. And even if you DO let it set up, almost every type has a Pokemon that can tank a hit from it, or deal with it another way. Fighting has Sash Breloom which can put it to sleep and also retaliate with Rock Tomb. Flying has Skarmory who can tank a hit and Whirlwind. Ground has Hippowdon who can do the same thing. Rock has Mega Aggron. Ghost has Sableye. Water has Rotom-Wash. Electric has a lot of things that can deal with it. Psychic has Slowbro. Ice has Avalugg. Dark has Mandibuzz which can Foul Play it and KO, and again, Sableye. And Fairy has Klefki(and Mega Mawile, but that's gonna get banned probably). Only types that I find might face a challenge against Mega Pinsir is Bug, Grass, Normal if you're not using BulkyRaptor or ScarfRaptor, and Poison if you're not using Weezing. And even then, the main thing is to not let it set up. Normal can dispatch Mega Pinsir with Staraptor since Quick Attack can't kill and Brave Bird WILL. Bug can use Flame Body Volcarona to try and burn it, or if that doesn't work, you have Scizor. Grass is the type that Bug murders all the time. However, you can always Thunder Wave it with Ferrothorn or Spore it with Breloom. Poison has
Another thing, there are Scarf Pokemon out there that can take a +2 Quick Attack and also retaliate back pretty hard. For example, Staraptor, Terrakion, Thundurus-T, Excadrill, Mamoswine, Genesect, Chandelure, Victini, Kyurem White, and Kyurem Black. There's also the matter of bulky attackers like Magnezone, Mega Scizor, and Landorus-T that can really put a dent in Mega Pinsir. You know what else puts a dent in Mega Pinsir? Stealth Rocks. Stealth Rocks means that it can't switch in and out, and even when it DOES switch in, it takes 25%(50% if you already Mega Evolved) so it really shouldn't be too hard to beat it.
Also, Mega Pinsir really can't set up too much. This is due to frailty on the special side, and the fact that Rock moves are everywhere. It can set up on anything that won't beat it up or status it, which is a generally small amount of Pokemon.
On the bright side, Mega Pinsir is still a pretty good sweeper. And while I think that's it's not broken like you've said, it's still a pretty very strong Pokemon.
P.S I might be a bit biased about this. I've always used Mega Scizor over Mega Pinsir. In all my tests, I've found Mega Pinsir to be kind of underwhelming. All these claims that I have made in this were based off of tests and a few calcs.
Here are the calcs:
+2 252 Atk Mega Pinsir Close Combat vs. 252 HP / 252+ Def Skarmory: 170-200 (50.8 - 59.8%) -- 89.1% chance to 2HKO after Leftovers recovery; This shows that Skarmory can live a hit and also how much of a killjoy it is to Mega Pinsir, Mega Scizor, and Mega Heracross since the others probably can't do more damage.
+2 252 Atk Mega Pinsir Earthquake vs. 252 HP / 252+ Def Torkoal: 282-334 (81.9 - 97%) -- guaranteed 2HKO after Leftovers recovery; Damn, Torkoal is defensive. Never really noticed until now.
+2 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Hippowdon: 318-375 (75.7 - 89.2%) -- guaranteed 2HKO after Leftovers recovery
+2 252 Atk Mega Pinsir Earthquake vs. 252 HP / 252+ Def Filter Mega Aggron: 144-169 (41.8 - 49.1%) -- guaranteed 3HKO
+2 252 Atk Mega Pinsir Stone Edge vs. 252 HP / 252+ Def Mandibuzz: 346-408 (81.6 - 96.2%) -- guaranteed 2HKO after Leftovers recovery
+2 252 Atk Aerilate burned Mega Pinsir Return vs. 252 HP / 252+ Def Sableye: 213-252 (70 - 82.8%) -- guaranteed 2HKO after Leftovers recovery
+2 252 Atk Mega Pinsir Earthquake vs. 252 HP / 252+ Def Klefki through Reflect: 190-224 (59.7 - 70.4%) -- guaranteed 2HKO; Since Prankster will allow Klefki to set up Reflect before Mega Pinsir can attack.
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 4 Def Staraptor: 258-304 (82.9 - 97.7%) -- guaranteed 2HKO; Still kamikazes itself to death, but at least Mega Pinsir is down, right? But in all seriousness, no Pokemon would want to take a Brave Bird from Staraptor on Bug.
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 0 Def Genesect: 204-240 (72 - 84.8%) -- guaranteed 2HKO
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 4 Def Chandelure: 211-249 (80.8 - 95.4%) -- guaranteed 2HKO
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 0 Def Victini: 195-229 (57.1 - 67.1%) -- guaranteed 2HKO
+1 252 Atk Aerilate Mega Pinsir Return vs. 252 HP / 252+ Def Landorus-T: 286-337 (74.8 - 88.2%) -- guaranteed 2HKO after Leftovers recovery; Landorus-T has Intimidate, so you can see how much +1 does.
P.P.S Hope it's okay if I flooded this thread with calcs. If not, then I can edit it.