I barely had any students from the PRC, several from ROC, South Korea, a few from Japan, and a lot from India.
The Indians I can particularly comment on. Sure, very rigorous educational system, at all levels. But very rote oriented. Just memorize a bunch of stuff, take a standardized test.
A lot of the ones I saw were superior students who hardly would have had a problem getting into a top school. (I taught Computer Science, so we got surprisingly familiar with some IITs. E.g., after we accepted a few grad students from one place, more would apply. The letters of recommendations would explicitly compare the applicants to ones we already had in our program. We got to know whose letters to trust, etc. If we had an Indian faculty member, that was a big help in understanding things.)
These were not rejects in any way.
After a bit we caught on: Great Indian undergrads sometimes don’t make great PhD students. They wanted to be told what problem to work on, what the answer should be and then take a test. But a thesis is all about developing a question and finding the answer yourself. And writing a thesis is a whole different thing from taking a test.
So we looked more carefully at applicants. In particular, what sort of research projects had they worked on, could we get a copy of their papers, etc.?
Korea and the ROC just didn’t have the quality of programs in CS, so going to the US was a must. Japan CS, OTOH, was pretty good. Far less interest in going to the US.
Computer Science might be something of a special case, for the era I taught, the US and a select few other places were the place you had to be if you wanted to really get to the leading edge. People need to be taught how to think.
But “rigorous” is not the same as “good”, in my book. An education needs to be complete. Mere rote memorization isn’t good once you get to the college level.