These are based on real people, and I believe the blending aspect, rather than starting from scratch, is intended to retain the physical plausibility between different face types, and keep things anatomically consistent. I’ve yet to see a MetaHuman pass into the uncanny valley in a bad way, which means it succeeds where needed.
The look you’re talking about is probably epicanthic folds, and the angle of the lower lid. More tools to push the bounds of each face would be helpful, but I’d guess that more face scans will make that less of a necessity.
Lastly, race sliders or selection might seem like the solution, but the fact of the matter is that everyone looks different, regardless of where they come from. No ancestry has a definitive look that can succinctly be categorized as “white” or “whiteness”, as these are traits and anatomies that descend from blending over time. Your friend has very unique eyes, or post processing done by his cellphone camera - an ethnicity slider won’t help there.
I think MetaHumans is a cool tool to provide a launch pad for creating characters. Being able to modify these characters post generation is probably going to be where it really shines, as I doubt Epic is going to turn this into something that makes every character for everyone’s needs right out of the box, but instead, something akin to a tabula rasa, a place to build from.