Army of Two has never been an especially bad series, it's just never been an especially good one. Providing simplified cooperative shooting action, as dumb and dirty as can be, the series has clearly found a fanbase for its particular brand of not very particular action. It's one of those series with a rudimentary purpose, and it fulfills that purpose without going the extra distance. Nothing acutely wrong with that, it simply is what it is.
That said, having had its third crack at the whip, Army of Two would do well to evolve or at the very least refine itself. There's only so much mileage in any idea, and when your idea is this spartan, there's not much gas in the tank at the best of times. Army of Two: The Devil's Cartel is EA Montreal's second sequel, yet it bears all the missteps of a first attempt, and retreads old ground in a less than forgivable way.
There's nothing wrong with a game like Army of Two, fundamentally. There's plenty wrong with this iteration of it.