Yeah, Mawile has eons of checks. 90% of the strong attackers in the metagame can check Mawile (Because Earthquake) and anything that can burn it or make it fall asleep can ruin Mawile. However, it has almost no counters; pretty much nothing can switch into one of Mawile's attacks. That is where its advantage lies. It's subtle, but the main reason why so many people got up in arms about it.
The trick with Mawile is that you need to consider the team it is in. It has the highest attack in the game, so if you can set it up to hit anything at all, you have an incredible advantage since it can knock out most bulk in a single hit. It is also relatively bulky defense-wise.
Mawile works best with Trick Room and/or Substitute + Focus Punch, so it pairs really well with Malamar or Whimsicott.
What I like to do is have a Malamar holding an Iron ball with Trick Room, Superpower, and Fling. Malamar uses Trick Room and proceeds to Contrary Superpower to entice the foe to KO them quick. (Or else Malamar gets a higher attack and defense than Mawile and wipes the floor with them) As soon as Malamar is knocked out, you have a set up Trick Room for Mawile to use it's slow attacks, and still be the fastest thing in the room. No priority move phases it (Except for Prankster Will O Wisp), and it can utilize the full extent of its strength; You just set yourself up for a Mawile sweep.
If Malamar is KOed early, you can generally sweep for the rest of the Trick Room, KOing one Pokemon per turn, and then one single turn after the Trick Room is done. (Because Mawile can generally take a hit)
This strategy works pretty well in the Maison too.
You just want to watch out for supereffective special attackers more than anything, but Substitute gives you enough buffer to KO a single would-be counter.