There are visibly serious problems with integration, which seem linked very heavily to the problems of dealing with unaccompanied child refugees.
https://www.spectator.co.uk/2016/09/how-sweden-became-an-example-of-how-not-to-handle-immigration/
This article describes, for instance, how difficult it is to support and integrate children who come without families (and how it is possible for young adults to pose as children maliciously) and how such children often end up in exploitable positions, either for sex trafficking or gangs. As far as I can tell, this seems far, far more likely to happen in the case of children (or young adults) than it does for more mature refugees.
I don't know what the solution is, exactly, but I'm not convinced that the solution is simply not to take in any unaccompanied child refugees because they are more difficult, as that seems certain to condemn them to death.