Adoptees often suffer a gnawing sense of betrayal, abandonment, guilt, low self-worth, lack of resolution, etc, which colours their entire life - simply because they were 'given away' by their birth mother and they wish on some level they had not been.
The problem is that while it's possible to objectively observe that an individual suffers from an issue such as low self esteem or abandonment issues, and it's possible to claim that "I think that these issues of mine were caused by by being adopted," that does not prove the that was the case. People can say "I think my XYZ issues were caused by [insert possible cause]" but that is not the same as proving that their belief is correct.
It's quite likely that if the hypothetical individual had not been adopted out, they also would have suffered low self esteem but would have attributed it to a different cause.
Certainly, twin studies and sibling-pair adoption studies (the ones which examine siblings who were adopted out versus those who remained with the birth mother) suggest that personal anecdotes about "my issues are caused by my being adopted" may be of dubious value.
Siblings who were kept with the birth mother have a high chance of suffering similar issues to those who were adopted out, except that the ones who remain with the birth mother tend to be worse off, on average.
No doubt these people attribute their low self esteem to other causes--"My mother [i.e. the birthmother] was a dysfunctional parent who parented me poorly and that's what caused my low self esteem and abandonment issues." Etc.
Again, and I'm pointing this out again, longitudinal studies of children who were created through gestational surrogacy indicate, so far, that they are turning out basically "normally" with similar psychological and behavioral profiles to those of children conceived the "regular" way among couples of similar demographics. If we are not seeing evidence that children of GS suffer psychological damage, it makes no sense to keep claiming "But they must do, surely!"