See MBS-4104 for an example of the problem. When merging, it would be helpful if there was a warning when trying to merge releases by merging mediums when the track artists don't match. That usually means that someone is trying to merge mediums instead of appending them, or is merging into the wrong disc. It should only be a warning though, not prevented entirely, since there are cases (e.g. composers versus performers) where you might merge two releases which are the same but are credited to different artists.