Your question catches me by surprise, so I decided to investigate.
Mega Maw gets wrecked by some of NatDex OU's biggest mainstays.
To my surprise, the most common sets it carries does not usually have Iron Head, and even if it did it'd be sacrificing the power it gets from Knock Off, Sucker Punch, or other coverage moves. So incidentally let's get that out of the way. I am basing this answer on common sets among the Smogon community. If you want to test a scenario with another move against any of the threats I list, be my guest and visit the calculator.
Heatran is the first problem I think of when I'm playing Megamaw. Assuming we're just going off of common Smogon sets, you'd have to be running Substitute and have caught your opponent with their pants down and have Substitute ready so you can drop a Focus Punch on it. The only other possible way to get out of that is if you Swords Dance yourself into manageable Sucker Punch damage; even then, to one shot it, you have to roll really high if you only have one set up. A Magma Storm from even a completely uninvested Heatran fries Megamaw, one shot.
Unless you run Thunder Punch, Toxapex and Corviknight completely wall Megamaw. Given that Mawile doesn't have any recovery available - although some vampiric sort of move would be cool - Corvi and Pex can just wait it out because it has Roost/Recover.
Obviously, there's no shortage of Fire-types in OU that are A) just flat out faster than Megamaw and tank the Sucker Punch like a champ and B) melt it. Even defensive variants of Volcarona one shot Megamaw. Fire is not a favorable type to either of its STABs - not that you usually see one of them.
Garchomp, while not a Fire type, also oneshots it in most builds with an Earthquake. (A tank would need to roll high for one shot.) Landorus-T also in most cases one-shots, but unlike tank Garchomp would be much safer if it had an unlucky roll. Even Great Tusk, the new king of OU Ground, well outspeeds Megamaw, not to mention resists Sucker Punch.
I could go on, but these are the big things that jumped out at me. I might be wrong about this last part here, but it would appear to me that Scizor might be the preferred Mega for the once-dominant Swords Dance job Mawile does. Maybe not any more effective against Fire types, but more durable against Ground.
