It is not that KH is hard to understand, it is that it is inconsistent and filled with plotholes. If you just close your eyes when it comes to that, like you should do for all games, the game is generally with its concepts as it is just about elements, friendship and fragments of existence leading to new existence that fights you again only to be reunited and all that stuff. If you approach it from a very analytical and logical point, especially psychologically you might as well spend your time differently.