Adoption and twin studies are very important for determining the impact of genes versus environment in the modern world (and hence the likely impact of various interventions). Other types of studies tend to show larger effects for some types of latter interventions, but these studies are seen as dubious, as they may fail to adjust for various confounders (eg families with more books also have more educated parents).
But adoption studies have their own confounders. The biggest ones are that in many countries, the genetic parents have a role in choosing the adoptive parents. Add the fact that adoptive parents also choose their adopted children, and that various social workers and others have great influence over the process, this would seem a huge confounder interfering with the results.
This paper also mentions a confounder for some types of twin studies, such as identical versus fraternal twins. They point out that identical twins in the same family will typically get a much greater shared environment than fraternal twins, because people will treat them much more similarly. This is to my mind quite a weak point, but it is an issue nonetheless.
Since I have very little expertise in these areas, I was just wondering if anyone knew about efforts to estimate the impact of these confounders and adjust for them.