I think cultural is probably the thing that has benefits, but they are so intertwined that they're essentially the same or impossible to extricate. Anyone who has visited Japan can understand that there is some benefit to racial/cultural homogeny. It doesn't really matter which of the two is the important part when they cannot be extricated.

Why Japan? How about the UK, or the US? Aren't those a bit more relevant? Those are certainly not racially homogeneous societies but why is it that racial differences are also cultural differences? Like, don't all Americans, black and white, love their guns for example? Or love baseball?