Their worst sin in my opinion is actually how they split off the end of the Smaug storyline to be resolved in, like, the first five minutes of the third movie (literally before the title) rather than resolving it in the second movie. Huge pacing mistake.
WHAT? Like, I know that the battle and stuff does happen after Smaug, so he’s not the final event of the book like you’d think, but I can’t imagine jumping into the third movie for a weirdly quick pay-off and then watching them drag out the battle for basically an entire movie.
God the more I know about these movies the happier I am in my choice to just cherish the book on its own.