Most of the stages are about three screens of increasingly difficult fights against standard enemies (of which there are probably around 10 species) followed by a boss. These can get very difficult, but you keep any currency you gained before dying, and there are plenty of optional levels, so you've got options if toughing it out isn't quite your style. Combat is theoretically pretty simple since you only have four attacks, but they do a fantastic job of shaking up scenarios to prevent you from ever getting comfortable with a strategy, and the skill ceiling is very high.
Normally these kinds of games are only really satisfying in co-op, but Full Metal Furies gets around that by letting you pick two characters in single-player. You can instantly swap between them at any time using R2, which opens up all sorts of interesting possibilities for comboing and balancing health. Enemies will often spawn with a shield that can only be destroyed by one of your characters, and those shields can change color as they take damage, so learning to swap well is absolutely necessary to progress. It's a great system.
I don't think anyone plays this genre for the stories, but FMF is actually pretty solid on that front as well. It has a very silly sense of humor that almost always worked for me, but everything is skippable if you'd rather it just stayed out of the way. Regardless, none of the scenes are very long.
Unfortunately, while I always had enough currency to level up both of my characters (Sniper and Engineer) once or twice after missions, I don't think it's possible to keep more than your player count up to level without replaying levels. I didn't have a problem with that, because I loved playing a team of all glass cannons, but mileage may vary. Also, the true ending is locked behind a sequence of clever puzzles that I'm not really convinced work with this genre. Finding all of the hints requires going back and meticulously exploring the backgrounds of older levels, which I found needlessly tedious. Again, mileage may vary.
I haven't found a co-op brawler that really grabbed me since Scott Pilgrim and Castle Crashers 8+ years ago. It's been a long wait, but Full Metal Furies is absolutely as good as, and perhaps better than, those games.