My general complaint is that there really isn't any nuance to any of the characters in the books. If you're thinking of complexity, you're probably thinking about the performances created by various excellent actors in the movies.

Also, none of the twists are executed well, as they are either deus ex rectum or they are telegraphed to the point where you don't even realize they were supposed to be twists.

There's no internal consistency within or between books. Macguffins come and go to serve the story, and every story is just repackaged public domain mythology.

Actually, the fan theory that all of it is the delusion of a mistreated orphan boy does tie several things together. The entire arc reads like the fantasies of a sad, lonely kid playing with a stick in his room by himself.