At least when children are adopted from dirt poor people in third world countries, this is less likely to be a problem.
There is no problem with adopting US children as long as parental rights have been relinquished and that the child is not tribal claimed (1/8 indian)
That said, one would want to look closely before adopting any child with a difficult genetic heritage of substance abuse or mental illness. It can be a very tough adoption. There are plenty of middle class babies available. Particularly now that there has been an economic downturn.
I adopted middle class babies, I told my adoption agency that I wanted children that were from families of similar construct.