I don't know, but I do think that if none of them had so far had any issues, then they've been very lucky.
I know (online) quite a few people who've adopted from abroad and whilst only a minority of them have children who have serious issues, the majority had a few issues - especially in the first year-2 years.
The initial adjustment for the babies in orphanages seems to be variable - many of them quite literally close down from the shock of being handed to new people and don't interact, smile or make eye contact for a day/few days. They're not like that normally, it was the sudden transition with no adjustment period
Longer term, I think certain countries have or have had a much lower prevalence of drinking in pregnancy and so FASD is rarer - I'm thinking China for instance here, so comparitively few Chinese adoptees will have long term issues related to foetal alcohol exposure compared to UK adoptees (and compared to Eastern European countries/Russia where many adoptees have FASD).
Orphanage care is variable, some orphanages are great and I would hazard a guess that many of your friends children were in good orphanages, whereas a bad orphanage with lack of care and neglect (unintentional or otherwise) will definitely leave many children with long term problems. It's important to remember that in terms of long term issues, serious neglect can actually (and often) be more profoundly damaging than physical or sexual abuse, because neglect will have a big effect on brain development.
On the other hand...good orphanage (or foster carer) from birth, continuity of carers, not being moved around, never having been seriously neglected or abused...that's the best thing possible after being taken away/abandonned/given up, and gives the best chance of a child doing well. But that's a chance/luck thing...it's never ever a guaruntee in international adoption. And it could happen like that in domestic adoption as well, with a baby removed from birth. So I think a better comparison would be not the country of birth, but the circumstances the child has been in. It's a myth that internationally adopted children are all easy and domestically adopted children have loads of issues.
When it comes to norms, I would have said that having some issues relating to, but not severe ones, would be 'normal' but that's only based on my own experience