As someone who always tends to overrate stallbreakers a bit it should come to no surprise to anyone that I find Mega-Mawile broken, more broken than either genie, more broken than Charizard-X, more broken than even Aegislash and the Deos (though not quite as unhealthy, but that's besides the point). Although not exactly the best Pokemon at actually breaking stall (that would be either Mega-Gardevoir, Mega-Heracross, or Mew) it's still extremely good at it : the SD set is vunerable to the WoW user but destroys everything else on Stall pretty much (especially paired up with a cleric/healing wish user, since the most common Wisper on Stall is XZard and that thing can't even take a neutral aka +2 burnt Play Rough, once it's gone you can heal up Maw and just rip through the team) , while the only thing that can take on SubPunch is MVenu who is pressured quite badly by it and needs HP Fire/EQ to even touch it. (Also, if you really hate Stall, you can always run SubPunch+Iron Head). Stall hasn't really been able to adapt to Mawile the way it adapted to Lando-I with SpD Gliscor, I mean you could point to Defensive YZard but few people can afford to run it on Stall due to the amount of support it requires (from what I heard you need 2 defoggers which is kinda ridiculous). Also it can actually lose to SubPunch if it comes in on rocks and loses Roost/Focus Punch 50/50s.
Now obviously, Mawile beating defensive teams isn't enough to make it broken, otherwise, we'd have suspected Mega-Gardevoir a long time ago. What makes Mega-Mawile banworthy is how much work it puts against offensive teams. Thanks to its amazing defensive typing, decent bulk, and Intimidate, it can eaily find an opening to come in vs offensive teams. Once it does that, it's very hard to deal with since it could either set up an SD and basically clean if all the things that can revenge it are weakened (and no, it really isn't "easy to revenge kill", all you need to do is get some damage on stuff like Keldeo and MGyara), set up a Sub which is easy given the switches it forces (basically guarantees a kill and a half, since nothing on offense can face MMaw behind a sub and live to tell the tale), or even directly attack. It's almost impossible to tell what it's going to do or even what set it's running, and getting it wrong will cost you way too much.
For instance, say you have Keldeo as your SD Mawile check, and the opposing MMaw just switched into Latias. Sending Keldeo directly into MMaw predicting SD is way too risky to even attempt, but the Latias you currently have out can't touch it at all, and you defenitely can't keep it in since you need it for the opposing Keldeo/Lando and you can't risk it setting up anything (especially a sub which basically guarantees 1.5 KOs) so what do you do? Fodder something else off? You could do that I guess, but once Keldeo comes in nothing stops Maw from switching back into Latias and abusing the entire team once again. Remind you of something? (Though this isn't nearly as bad as Aegi since MMaw can usually only come in twice vs offense, but that's well enough to score 2 KOs, possibly even clean late-game). And yet, as threatening as this thing is in theory, it's somehow so much better in practice... The only time I saw Mawile do nothing it was a Shed Shell variant, so...
There simply isn't a matchup where MMaw will not preform. Against defensive teams it can spam Play Nuke, against offensive teams, thanks Sucker "Stronger Priority Than Silk Scarf Ekiller" Punch it can revenge kill stuff as well as preform the role of a late-game cleaner, and one that is impossible to switch into no less. Above all, I believe MMaw to be broken simply because it defies the risk/reward that usually comes with wallbreakers. Usually, wallbreakers become less efficient the more offensive the teams they face are. That how it should be, since there needs to be a deterent to just filling your team with the nukiest stuff possible. You get to improve your matchup against defensive teams, in return you become more vulnerable against offensive ones. MMaw lets you improve your matchup against defensive teams, and also improve your matchup against offense as well. MMaw is just far, far too low-risk a wallbreaker to not be considered overpowered.
I don't know if it's because my obsession with including Latias on most of my teams, but I've had to alter almost all my teams to deal with MMaw better since I always end up struggling with it no matter what playstyle I use. A lot of these changes (stuff like putting Wisp over Toxic on Heatran, swapping Scarf Lando-T for Phys Def, I even nearly gave Sub to Sand Rush Exca) actually make me worse off against most of the metagame, but it's worth it simply because I don't lose to MMaw as often. I think this is evidence of teambbulding restrictions caused by MMaw, however, that's just my experience with it, it may not be the case for other people. Another annoying thing about MMaw is that you need a solid check to both sets. For instance, I had a team with Wisptran Lando-I MScizor and Scarfchomp and I still struggled with the thing. And unless you run something like Phys Def Lando-T or Arcanine/Weezing if you're into that, you can't reliably check both sets with the same Pokemon. This isn't really a case of "Mandibuzz effectively switches into Aegi 70% of the time but gets nailed by the rare Head Smash", since both these sets are, from what I've experenced, almost exactly as common as each other, and you can never tell which one it is. It's a litteral 50/50 as to whether or not Wisptran is going to stop that particular MMaw, as opposed to Mandibuzz who, in practice, can usually switch into Aegi.
Speaking of 50/50s, I honestly believe MMaw causes more of these than even Aegislash (whether it's going to setup or attack, whether it's going to SD or Sub, whether it's going to Sucker or Play Rough), but I won't bother you with a lengthy paragraph on that since Jukain is probably going to do it/has already done it anyway
Oh, I almost forgot to mention this but MMaw is really easy to support and make even more threatening as a result. I guess you could argue that this applies to almost any sweeper, but it's particularly true for MMaw due to the sheer number if things that can turn it into ecen more of a monster. Screens, Trick Room, Sticky Web, even SmashPass can make it an absolute nightmare for both defensive and offensive teams (not that it wasn't in the first place)
Funnily enough, a couple of months ago I didn't think MMaw was even suspect-worthy. But the more I faced it, the more and more broken it appeared to me. Now, I consider it to be the most broken Pokemon in the metagame at the moment, by quite a margin.
BAN
