They are terrible and cause players to not care about the community on a server, knowing they can just leave it if goes to crap because of behavior. So its a player issue, enabled by blizzard.
They seek a balanced place to play, and the only reason the target server is balanced and thriving is because the native community created that. Then the transfers come in, and if they don’t assimilate to the social norms of the host server, the host server goes to crap as well and the cycle starts over.