Let me preface this by saying that I read all the books, in English and in Chinese, and that I've watched the Tencent adaptation. Whatever qualifications these "book-fans" have, I also have.
Most complaints I see are variations of the following:
1. They switched Wang Miao into "Oxford 5"
2. They race-swapped or gender-swapped important characters
3. It doesn't feel as "sciency" as the books
4. Platonic relationships from the books turn physical on screen
If you want to watch a 1:1 book adaptation, there's the Tencent one for that. Quite frankly I found it boring as a TV show. Some source material just don't translate well to the screen, and 3BP is one of those. A single episode of TV is only 1 hour long, so you can't have a middle-aged male stoic as the sole lead character and follow all his scientific ruminations, it just doesn't work for people who haven't read the book. The Tencent series was faithful, but to the point of being zealous and therefore did not produce an enjoyable standalone TV series.
In this Netflix series, the showrunners made some creative adaptations to address these problems. I think splitting Wang Miao into a character ensemble that includes future main characters was a smart idea. This injects a "character core" into the story that was missing in the book series. Without this core, we won't have any continuity across the 3 books.
That said, the character building for the Oxford 5 left a little to be desired. I want to feel a little more attached to the main caste, but in the end I felt more connection to Ye Wenjie. Obviously the pace of plot points is quite fast so it's difficult to weave in character development that are not related to the main story. I'm not sure how they could have done it better but I wish they did.
As for gender swapping... what's the big deal? Does a female nanotech startup founder in this story make people that uncomfortable? Is it really THAT jarring? If this somehow wounds your pride... lol. I work with lots of extremely smart women in STEM so I don't find this unrealistic. By the way, in China the percentage of women with higher education is higher than men.
I'd give this series 4/5, but seeing some of the braindead reviews on here, I'll add an extra star.