I dunno if Mega Mawile will still be on topic when I'll finish to write this, but while I think it is one of the best mons in the tier and that any semi-serious builder / player should prep for it, it is not broken imo. I believe that broken in the current metagame would be more akin to something like Magearna, who can screw you over with its sheer number of sets and thus, its unpredictability. For comparison sake, when building you have to prep for Specs Magearna, Kiss Power, Heart Swap / AV or CM Split depending of your team ( if you don't play teams reliant on Clefable / Mega Latias sweeps - so Balance usually - or Stall, these ones won't often do much and are not too hard to check with stuffs like Landorus + whatever outspeeds Magearna and can finish it usually though I'll give you that ), SG with its various coverage and Z moves, or hell even surprise Scarf or say, Trick Room. I even heard about Iron Defense and physically offensive. The amount of times I've been swept by Mag just clicking SG or reversing the course of a game I thought I won just by using TR when I thought it'll be SG and so die in one hit, I'm sure most of you can relate. And I don't play that much nowadays so I didn't meet Specs a lot but the few times I did it was really annoying ( the opponent was like " oh, you like to try and predict to avoid my attacks by switching accordingly ? I'll Trick ya. " ) Good luck trying to prepare against every sets. But Mega Mawile has what... three sets ? One of which you guys may not know too well since I don't recall seeing any of you talk about it ( Sub 3 Atks, usually Ice Punch / Play Rough if you want to beat Mega Sableye without the need of Electric Terrain support or chip damage / T Punch / Focus Punch, this set is a lure that came out in late gen 7 to not only lure Heatran, Landorus etc., but also to put a huge dent on Mega Aggron's health since it was used in favor of Mega Sableye on Stall to beat Mega Mawile teams that ran rampant at some point. In Nat Dex it seems it hasn't been " discovered " yet - cause let's be real, most " innovations " of this meta come from new moves or gen 7 OU, Specs Mag was used in the beginning of SM, the spreads I can see in the Contribution & Correction thread are often gen 7 spreads, early Nat Dex people were like " with 8 Spe on Pex you can outspeed Reuniclus huhuhu " wow mate that's new, what a genius you are for figuring that out lol more recently I even had someone in the Showdown chat trying to tell me that SD Synthesis Kartana wasn't from gen 7 when it was either a blatant lie or someone showed it to the (wo)man and (s)he didn't know where it came from, I digress tho - ) ? Come on, it's not that hard to prep for or to beat. There are a multitude of viable, splashable mons and techs to check this mon and you know what it's gonna do. And if we talk " broken " as in " too powerful ", Mega Mawile is inherently slow, so any mon ( quite a lot of them actually : Ttar, Lando, Chomper, the Grens, Koko, Lele, Heatran, Zapdos, Mega Lopunny, etc. ) that can survive +2 Sucker Punch and that can KO it in response ( sometimes you'll need chip ) can deal with it, granted said mon will lose a lot of its health in the process most of the time but it will have done part of its job. And yes, a problem with SD is the fact that you can't really afford to take chip damage with common offensive checks, but that's why you should use at least two of them or one and a defensive check ( the hard part is keeping them healthy though if the opposing Mega Mawile is kept to sweep late game, you better force your opponent to bring it in battle so you can deal with it as early as possible most of the time, make bold plays attacking it when it tries to SD in your face so you can beat it or revenge kill it easier, etc. ). SpD Heatran with Scarf Landorus, what's a Mega Mawile ? Of course I'm exaggerating, it's often not as easy as I make it sound in practice, but like, Mega Mawile is slow, it won't kill everyone with +2 Sucker Punch, it's rather frail and to get at +2 it has to find an opportunity to setup, not get burned, not get beat, so getting kills with it is not as free as say, Final Gambit Scarf Victini or an unexpected SD Lando that nukes one of your mons, and we have mons like Mega Medicham, who has virtually no switch ins ( bar what... unseen Shedinja ? ) especially now that it tends to run Thunder Punch ( which let you beat Slowbro and Reuniclus, Mega Sableye etc. with Electric Terrain ), yet we not here thinking mon should be banned when just like Mega Mawile, the only real counterplay against it is revenge killing ( I know that SD is better and harder to revenge kill, but beating these mons is a similar process ). Basically, these mons can easily get kills, but it's also somewhat easy to kill them.
Additionally, I would like to add the fact that more often than not, when your team is prepared against SD Mega Mawile, it is also naturally prepared ( arguably even more ) against the AoA and the Sub set ( the latter which you sack a check to beat the Sub and you revenge kill with another check, which will be easier than against other sets since it will be weakened ). The former set typically runs Play Rough / T Punch / Sucker Punch / Fire Punch / Brick Break etc. ( you could add Iron Head for Mega Venusaur or Ice Punch for obvious targets ), but most defensive mons not 4x weak to Fire / Ice Punch won't die in one blow without SD. You rarely use one defensive mon, usually you have a Lando in the back too so you can switch it on T Punch to Intimidate it for example... things like that. Another thing to note is the fact that it's not uncommon for common Defoggers like Lando or Zapdos to have a super effective move against Mega Mawile.
Lastly, Spacio, I'd recommend you to use Mega Aggron / Melmetal with Wish support for SD and / or say uh... Will-o-Wisp Moltres ( that you maybe forgot to mention as a check to AoA, theotherguytm. Btw Skarmory can also somewhat check Mega Mawile with Iron Defense, Counter and whatnot, I'm not too familiar with these sets tho but my Mega Mawile got beat by them by surprise in gen 7 iIrc ).
Sorry for the long ass text but all in all although I could be wrong this is my ( hopefully valuable and useful ) opinion, TLDR yeah Mega Mawile is very strong but imo if you build teams solid enough to check it ( two offensive checks / an offensive check and a defensive core with a Ground type to switch into T Punch, or whatever works for you and isn't bad ), don't get chipped against SD so you can revenge kill Mega Mawile in a pinch, and force out the mon to beat it as early as possible, you should be good if you don't play like Ash.