I can't tell for sure who can't get a job, but i have seen only Koreans, Mexicans, Europeans (most flavors), N Americans. Shocking to me, I heard China Southern hired a bunch of ex JAL Japanese guys. I though they hated each other.
I have personally seen no pilots from anywhere else in SE or S Asia. None from Africa. Surprisingly to me also, only a few Brits and Australians. Not sure why on this one as they usually outnumber Americans and Canadians in contract jobs.
When you get hired in China, you are initially getting hired "by China" not the airline. Your screening is starting the process to get a Chinese ATPL. The standards are probably the same for all the airlines as the CAAC is doing the screening.
Part of the process is a security background check for the CAAC crew ID card. From what the Chinese told me, if the CAAC can't check every single box, you won't pass. I saw a couple pilots fail this process. It could be that many pilots can't get a job because the Chinese already know that the particular country can't supply their required documentation for this part.