I think surrogacy has the capacity for some horrible outcomes in a different way from adoption. I cannot imagine what it would be like to think you were fine with surrogacy and then experience life-threatening complications, or become disabled. In the case of adoption the risk to the birther is already in existence or already happened, it didn't come about due to the desires of the adoptive parents.

It also matters where they live and whether a surrogate could get appropriate care there (e.g., for a late miscarriage or fetal demise).