Not broken or unplayable, just bad. Stay away unless you're prepared to spend over $100 per week just to have a chance to keep up.
The game is a desperate cash grab. Unless you have at least one "fully suited" 5 star hero (5 star is the best in the game, fully suited means owning at least 3 different suits and thus access to 3 special abilities in round) and spend over $100 each week for the new damage multiplier heroes and energy, you won't stand a chance.
The leagues and leaderboard system is designed to force you to spend real money for more energy to play more rounds. Then be prepared for lopsided matchmaking that will stick you with the same player stomping your team into the ground every round for hours and hours and hours. And if you don't place in the top 20 of the highest league, you'll miss out on one of next week's damage multiplier hero, setting you up for extreme failure and disappointment all over again.
Advancement is painfully slow if you don't spend real money and lots of it at one time. In-game currency is the only way to get 4 star and 5 star heroes unless you can place in the top 20 of the highest league each week. When a decent hero is up for grabs, players spend lots of real money and more than 60 hours per week.
Your best bet for the in-game currency is the daily logins. Every two weeks you earn enough for a premium spin. The likelihood of getting anything but a 3 star hero is astronomically against you, especially drawing one of the highly coveted 5 star heroes that you'll routinely be matched against. You'll be outgunned, making the crawl towards the other source of free in-game currency from high scores long if not impossible.
There's the engine for a simple but fun game underneath the pay or die mechanics. The touchscreen controls are simple and intuitive. Rounds are fast and frantic. Improving your heroes is streamlined unlike the complex systems of most other freemium mobile games. You fight to gain XP and level up, collect new suits from premium spins or weekly competitions for new special abilities, and improve your special abilities by collecting duplicates of the suits or using ISO-8, the only other currency in the game.
However, because of the complete reliance on spending huge amounts of real money each week just stay afloat in the game, I can't recommend Marvel Mighty Heroes. Try Marvel Future Fight instead.