It simply isn't broken its just a very very solid team-up.
We really need to see MK in the meta. No doubt people will want to create all the cool team-ups in the world, but there have to be some limits.

the fact that UDE made new formats is proof that they saw a possible upheaval with MK.

We'll just really have to see.

Midnight Sons does exactly the same thing other team-up cards do, minus the fact that you need to control a character of that affiliation. It is detrimental to try and team-up with three or more teams with Midnight Sons, as it will just make a highly inconsistent deck. You will usually just use it so you can use ONE team's support cards.
This is unlikely to happen:
Daredevil with Fantasticar and Elektra with Beta Club will team attack, I play Bamf! followed by It's Clobberin' Time on Daredevil and Astro Force on Elektra. I will then play Fizzle on your Acro Dodge. Your character stuns. I play Teen Titans Go! to ready them both. Exhausting Elektra to play Kidnapping on your guy. Daredevil attacks another guy and I will flip up Lost City and power him up three times.

yes Midnight Sons is broken but MK needs a little kick to it.
Once people see that Midnight sons decks don't work out as good as others they will fizzle down, so let us have our fun with our Spider Knights and Marvel Underworls, and Crime Knights