If I understand correctly, some of the changes are because Netflix wants to take the series in a somewhat different direction than the books, and write their own independent stories for the Witcher world and its popular characters. This very understandably has some people who are fans of the books fired up. Those characters already have stories, and the way a lot of essential things in the lore of the world work are already established. The series is partially relying on those existing stories and lore for its current stories to make any sense, but is also altering them as it goes. That's a tough pill to swallow.
I've never really thought of this book to TV conversion as being like GoT. It seems to me more like the Shannara television conversion, which thankfully went up in flames. (sorry but not sorry Terry Brooks -- you shouldn't have sold your literary soul to MTV).
I think the Netflix version of the Witcher is vastly better than Shannara, but I'd be shocked if it ever achieves anything on the level of GoT acceptance. The scope is just too narrow, being focused around two or three characters, rather than an epic conflict in which the characters are mostly storytelling devices. However, I think Netflix is hoping for something big, and the changes are what they think will give their expanded story lines broader appeal. I'm skeptical, but I guess we'll see.
All that aside, I enjoyed the first season, and have read that the second season is better. So, I'm looking forward to it.