Sherry is a good character, but I think Ashley had some potential. She sure showed that she can handle RE4, so they could have matured her in RE6 or even RE7. I wish they wrote her better in RE4 though. She wasn't terrible, but could have been a little more developed. Her slight hint at a romance with Leon should have also been explored more.
As for Elizabeth, I'm completely opposite of you. I love Elizabeth and I found her a very interesting and compelling character. I also liked the bond she developed with Booker that was never hinted at any romance bull****. Not only because it would be like cross-universe-time-incest(whatever?), but also that romantic relationships between protector and protect-ee have been overdone. Really like the way she was written and I thought her voice actress did an incredible job, I never even thought about the voice acting aspect until I read you post.
The whole voice acting and acting in general criticism is a dodgy territory to me because to me, they are a good actor if I never questioned whether they are acting or not and I rarely question such things. Although there are performances that I sometimes find a little over the top or perhaps underwhelming (resident evil 1). Generally speaking though, most actors and voice actors do fine. Video games have a bit of difficulty as we tend to be more critical of these worlds and have thin suspension of disbelief.