Three things jump immediately to mind.
First, the ham-handed strokes to establish the heel/face injustice to resolution cycle that was meant to drive the book’s tension. It was transparent and felt like bad episodes of American professional wrestling.
Then the way the girl single-handedly invented much of the industrial revolution. Amazing concepts like “what if I bought wool direct from the ranchers so I could buy low and sell high?” And no one had ever thought of this in the history of wool commerce. I don’t remember the details but I remember my suspension of disbelief crashing repeatedly at the way three or so main characters came up with a continent’s worth of societal developments.
Perhaps biggest was Ken Follet’s concept of romance. Roughly verbatim:
Girl: “I’ve never told anyone this, but I was violently raped in front of my little brother as a teenager.”
Boy: “Oh. Well, if you don’t have sex with me also right now it will really hurt my feelings.”
[sex happens]
Just my take. It is of course tremendously successful and tremendously popular.
(Also my take, if you enjoy the satisfying establishment of heel and face, and the ultimate resolution, Conn Iggulden does this deftly in his Caesar and Genghis series. I also appreciate his historical responsibility, including the postscript about where he was guessing and where he changed known facts for the narrative).