The worst problem with controlling the genetics of _any_ population is that there's always going to be some trait which we don't fully comprehend the importance of. Obvious examples:
- Sickle cell anemia: considered a health problem to someone living in an American city, but elsewhere can provide life-saving resistance to malaria
- slow metabolism: while this makes you more likely to become fat/obese in a food-rich country spending your time indoors, you wouldn't want to be skinny if you had to hunt for every meal, and lived in the Canadian arctic (even with a Gortex coat you'd be at a disadvantage
- low IQ : there are plenty of jobs out there that normal/below average people are comfortable with, but the very intelligent would find unbearable.
And then, there's skin pigmentation: too little and you burn in the sun and need to wear sunscreen/stay inside, and too much and you produce less vitamin D and need to adjust your diet/stay outside.