Book 4 was a difficult read because the antagonist is ridiculous. His motivation was “the company” without any real reason for it beyond a paycheck. No one is that blindly loyal without a personal reason, it was dumb. Up until this character, we had nothing but rich characterization. This antagonist felt lazy and 2D, making the book difficult to get through. An entire antagonist built out of tissue paper.
Then, in later books, we sort of lose the crew, to old age, so we have to rewire our brains to accept them in different roles, as old people. Aging changes the landscape. It’s like that last season of Fringe where you’re questioning what you’re watching. You even check the vid to make sure it’s actually Fringe. Later, you accept it as Frimge, but it’s not immediate.