It tends to be the 'easy to place' babies that bring the biggest challenges.
By the time a child is in school generally (not always!) their issues are largely known and potential adopters in a good place to make sure the right support is in place before getting the adoption order. I know several adopters who had a year or so between placement and applying for the adoption order to make sure that all assessments, support plans, EHCP etc were in place. NOT because they had any second thoughts, but simply because that was the point at which the statutory services had legal responsibility to act and so the adopter was in a position to insist that these were done.
A baby has a lot of unknowns in terms of their needs, and while some will have minimal additional needs than any other baby, things like foetal alcohol/other substance exposure or the genetic aspects of severe mental health conditions (for example) are very difficult to predict and can have a major impact.
DD1 as an adult is pretty much who she was at 10 (via a very intense period in her teens when she was struggling to come to terms with the way she was let down by so many adults aged about 4-10). It helped that those early years were stable and her birth mum was in a good place in her life.
DD2 was an easy pre-schooler meeting all developmental milestones, whose needs became apparent when she went to school, then there was a nightmare period of getting those needs recognised. Finally she has an EHCP, is in a school who are very positive about supporting all their young people and value her as she is. She is now beginning to catch up - but has only just at 15 been assessed for ADHD, which has been on the radar since she was 6/7.
I know quite a few adopters of school age children, and with one exception they all say that things have turned out pretty much as predicted. Some ups and downs, of course, that's parenting. But no major surprises.