I've just started playing 15 and I can only assume the chaotic combat is deliberate and intentional, with it's ragged back and forth attack charges, having to drag your injured bros out of the fray, and mainly being able to hit your buddies with friendly fire by accident.
It is like the game is trying to simulate being in a brawl with your buddies.
Compared to the orderly group combat of the main characters from 13 on down, the main characters in 15 are an unruly mob of disorganized yahoos.