I have not adopted myself, but have a close family member who is adopted, as well as class mates, friends of family etc.
None of these have had any major behavioural difficulties as children. My family member who is adopted has been a totally "normal" child who has done well in school, had friends and boyfriends, a university education and good career.
Also the other adopted children i know of has done well in life so far ( have partners/families/good jobs).
Now, of course i cannot know about any emotional problems which they have kept to themselves, but with regards to your question about major bahavioural problems and big problems handling a regular life, I have not seen anything like that.
Now, in my country, domestic adoption is almost non-existant. The adopted people i know therefore come from abroad (so usually have different skin colour to their parents), ususally from orphanages, at the age of 1-3 years (some older).
I have no problems understanding that children who come from more extreme backgrounds in the UK (neglect, drugs) might have bigger problems. Children with that kind of background are usually fostered in my country, and many have emotional and other problems. But what people seem to have been telling you is that almost every adopted child will have some major problems. This i find a bit strange.
According to research in my country about 70 percent of adopted children will do well, while 30 percent will have warying degrees of difficulties.
That is appearantly the same as in the general population, but within those 30 percent you might find "worse problems" than in the general population.
(a summary in english here www.comeunity.com/adoption/adopt/research2.html
, see under Most people manage well"
Just to emphasise that I realise: for the people whose children are troubled, positive stats obviously wont help, and I agree as parents you have to be prepared for huge challenges. Adopted children will probably all require more emotional support, if that are the right words to use (aplogies if I use some english terms wrong in this post) . Still, at least on a purely anecdotal level, I can say that in many, many cases, adopted children will go on to thrive.
It would be interesting to see british research on this topic. Is it really so that british adopted children are much more likely to have problems than scandinavian adopted children? Or do the domestic vs international adoption difference muddle up the issue ? ( I have a feeling this might be the case)
Again, I want to stress that i am not trying to say that adoption is a completely fresh start and that you shouldn't expect any issues, but at the same time i can't see that it is right to say that most kids will have big problems.
or have I compeletely got the wrong end of the stick? maybe my outlook is just coloured by seeing how well adoption has worked in my family, and therefore have grown up with the view that adoption is just a different, but equal way of having a child?
I'll stop waffling now, but want to wish you good luck in the process!