No matter how granular you get with specific ethnic groups, it’s not possible to capture the long tail of all the types of people who exist, and all of their appearances.
If you ask Midjourney to draw a man, should he be wearing clothes? A man might be naked. Should he have two arms and two legs? Some men don’t. What about two eyes? What color skin should he have?
The fact that Midjourney will never draw a third degree burn victim when simply asked to draw “a man” isn’t a flaw in the model. The model is biased, yes, but it is biased towards utility.
For me, the salient observation is the complete lack of any kind of creativity or anything approximating imagination, of those models, despite a constant barrage of opinions to the contrary. Yes, if you asked me to draw you "a mexican man" (not "person") I'd start with a somberro, moustache, a poncho, maybe a donkey if I was going for a Lucky Luke kind of vibe. But if you asked 100 people to draw "a mexican man" and it turned out they all converged on the same few elements you'd nevertheless have 100 clearly, unambiguously different images of the same kind of "mexican man", often with the same trappings, but each with a clearly distinct style.
It is this complete lack of variance, this flattening of detail into a homogeneous soup, that is the most notable characteristic, and limitation, of these models.
And yet when hands came back with beautiful variations in finger count, people were unhappy.
Offspring tends to go in one direction or another so one group may end up inbred, but a mix is more accurate than compiling the beliefs of human cultures about their genetic traits and isolation from each other.
Inside cities, no one is specifically looking for a member of their own tribe to marry so the ability to identify ethnic groups by facial features is collapsing.