Chinese academia is way too competitive and exhausting for your American scientists. How many American grad students and postdocs are willing to work 6-7 days a week and have their lab meeting held on Sunday, which is becoming a common occurrence now?

These are some of the most hardworking people out there (and you can see them carrying their work ethic overseas as grad students and postdocs in Western universities) and churning out top tier scientific publications every year. These are the people you are going to compete against. If you think the “publish or perish” pressure is bad in Western academia, then in China this is ramped up to the next level.

Finally, connections are very important in China, even more so than in America and Europe. Much of the institute’s funding is going to be received by the heads of the departments, who then distribute the funds to the individual labs underneath them. Unless you are already a well-renowned scientist, you are going to be competing against your peers who have actual connections to the senior figures in the department.

Chinese science is very well funded, but the competition is also extreme, just like most industries in China. You don’t publish well, you won’t survive. Europe may be lacking in their funding, but their work culture is also more relaxed and suited to Westerners.

I have several friends with PhD who emigrated to the US and loved it there because, according to them, at least it is nowhere near as exhausting as they had it in China. They actually have the time to enjoy their weekends, a luxury you won’t get in Chinese academia.

US scientists escaping fascism to Europe just to be hit with the exact same shit in five years is gonna be a funny bit

It is a safe assumption that a majority of current US scientists who leave will most likely go to Europe, yes. But long term that's a relatively small piece of the brain drain issue compared to which countries will end up producing more (and better-educated) scientists in the coming years. It's not just a matter of the ones that already exist going elsewhere, it's that there is so much less incentive and ability for a person to become a scientist in the US than there used to be while there is significant incentive and ability to become a scientist in China. I expect the incentive in the EU is also going to rapidly deteriorate, so the influx of US scientists there is just a postponement of brain drain in the west as a whole.