Like I said, hard to pick just two.
Aventus's unadoptability is perplexing. His story is just as sad as the other kids. My theory is that the Hearthfire team just forgot, since it makes NO sense that they would choose to leave him rotting in his home. And the developers might be fallible, but they're not dumb.
It might have been a simple glitch. If so, an Aventus patch may be in the works as we speak.
Aventus has a unique voice actor; from some leftovers in the file, they did want to have Aventus be up for adoption but couldn't get his actor back.
Aventus' VA sounds a little older, and is a marginally better actor. However, the child actors have matured a bit since the vanilla lines were recorded, so the difference is less distinct now.
And of course, you can't force someone to act for you, and we don't know what their contracts entailed (it's possible, for example, that the vanilla actors who came back for Dawnguard and Hearthfire were bound by their contracts to act for Skyrim and all its eventual content). The other option would be to get an entirely new actor to re-record all his vanilla lines, which means essentially paying twice for the same lines, all for a relatively minor character.