I think its not about race, it's a cultural thing. People are stronger when they live in an environment with a homogenized culture. They all understand each other better, understand rules/laws/customs/morals....they have more unity. They have a sense of being part of something that's bigger than themselves.
China is extremely homogenized, and they don;t have widespread social problems like the US, and they certainly aren't actively seeking to diversify their population, it is unwise to do so.
The unity part of China comes from the collectivist culture versus the individualistic culture in states. It has nothing to do with what your talking about. Ray Peat talked about this in his book Mind and Tissue. Americans make every excuse in the books. Blame minorities for their economic failures. The widespread problems are cause by you yourselves.