They butchered the essence of the characters. Sure you can make your own thing with the show but when you start changing the way the characters act and the motivations they have, you run into problems. Yennefer would NEVER do the things she did in the second season, the same goes for Vesemir. I still don't know why Fringilla is a main character, she says 5 sentences in the first 5 books. Her biggest contribution before the last book was she temporarily blinded Yennefer at the Battle of Sodden.