Gotta love these social workers who bombard you with adoption horror stories and then question you if you express any concern. I got this when I dared to ask about the possibility of FAS when I met the medical advisor - her response (to the SW, in front of me), was, "This woman just wants a perfect baby. I doubt her suitability to adopt."
Look, this is really hard stuff to convey with the right kind of balance. Everybody travels hopefully to parenthood. The vast majority of us adopt because we want to start or complete our families - not because we're saints who want to sign up for a lifetime of therapeutic parenting. The reality is that most adopted children have some kind of problems, but of course there's a whole continuum and few pose very real threat to their families. And you love them, they're your kid, so that helps you get on and deal with things.
I have to say, though, that adopting when you already have a birth child does need more consideration. I did it, and the social workers happily glossed over the potential problems, and I was happy to be glossed because I wanted to hear that everything would be ok. 7 years on, everything is not ok. I think there have been huge benefits for my birth child, and huge costs too (dd2's needs take over the family, and yes she is first victim of dd2's rages and physical aggression - though it's not serious in intensity). They do love each other, and after we're gone they'll have each other, but I do think that dd1 would have had a calmer childhood with more attention for her own needs if she had stayed a single. And I do wonder if dd2 would fare better if she had our undivided attention. Time will tell.
I do feel for you. I remember desperately wanting some kind of reassurance. Nobody can give you that. But I think many adopters would say (and have done on this board) that they started out hoping for normal family life, got something a bit different, and readjusted their ideas of 'normal'. A minority of adoptive families have no problems, a minority have devastating problems, and most of us have more to deal with than non-adoptive families, but we love our kids and our families are stable.
One last thing: I'd caution against thinking adopting a young one ensures no problems. dd2 was 10 months when she came to us, and she definitely her problems - insecurity, low emotional regulation, physical aggression, overwhelming grief for the family she has lost. But she is also bright, funny, warm and loving, and doing well at school. I used to underestimate the impact of early life trauma - I don't any more.