Race mixing destroy people. People evolved in their lands for thousands of years, and introducing foreign blood will cause mismatch with the land characteristics. E.g. Tibetians have evolved high lung capacity due to high altitude, but if you thrown in say, people from flatland into the genetic pool, the next generation would struggle to breathe.
Culturally speaking...
The lack of a organic culture/identity.
The best and most effective way to unite people is through their race which cannot be changed, homogenous societies have higher social trust in general.
If that is not possible, there is the cultural/state identity, e.g. Singapore. Usually these countries have a limited pool of mixtures to choose from, e.g. Singapore only have Indians, Malays, Chinese. In a twisted sense, multi-ethnonationalism can work (think ethonationalism but for multiple DESIGNATED groups)
Next is religion identity, which can be seen in Middle East. Religions are highly vulnerable to corruption/interpretation of the text, forming blocs/sects, that produces some of the most brutal rivalry still see today, such as Shia vs Sunni.
Finally....
Despite the iberians' strong Catholic views, it failed to prevent mass degeneration of the Latin countries (including crimes). Because these countries lack what is needed to organize the people.