Other people have mentioned the risk of class distinctions being codified at birth. I worry about that, but I think there is also a larger concern that we still don't really know what we are doing here yet, and we can be selecting out traits that are beneficial for Human flourishing (in the species sense) in our ignorance. There are plenty of examples of disease resistance that look maladaptive, sickle cell and malaria being the classic one. We have no idea what we're doing and we need to take this cautiously.