Seems like they have a lot of trouble replicating hands & feet. Why is that?
I would assume it's because in reference images, hands/ feet are not always prominent or in focus.....
i.e..... the image is focused on the face/bust of a person.
Also people can have their hands in soooo many different positions. So the lightning/shading can be weird. Since AI isn't actually "seeing" the images it's replicating, the weird shading/lighting/positioning is what comes out in the renders.
This article states, ai doesn't really "know" what a hand is to be able to manipulate it and render it all the various scenarios. It just mixes a cluster of reference images and voila..... however I think that is the very simple answer to a very complicated issue/process

Why AI-generated hands are the stuff of nightmares, explained by a scientist
Whether it's hands with seven fingers or extra long palms, AI just can’t seem to get it right.