They did not steer from the source material. They rushed it thinking a whole season could be condensed into a rushed 90 minute movie with a focus of stoicism.
They had some really awful fight scenes, and you can actually see the minions standing around waiting for their cue to move.
Then there was weird BTS actor issues. One of the sponsors insisted that his daughter be given one of the leading roles, despite that she could not act the role. This meant they also had to recast the guy playing her brother so they would be the same race. This also meant all four main characters were now white, so they recasted another of the main characters with an East Indian. All the supporting characters from Fire Nation were then recast to match his race. So all but one key character was recast, and he was cast for his stunt work instead of acting ability to interact with green screens.
A movie about the production would probably be better than the movie.