The low-cost activated ability makes drawing cards inexpensive and the triggered ability upon 8+ stationing is a passive and casual win condition with the land sacrificing engines Hearthhull promotes
As for the two big decks, I guess you could ban Emrakul, the Promised End from GB Delirium, but I'm not even sure how much that would matter
The title of most expensive means nothing if no one will buy the card
Of course, as I always say, a card is only as good as the context that surrounds it, so if there are not that many good fighting attackers, Barbaracle wont be as strong but I honestly think it is a fantastic option that immediately makes new fighting types a greater threat
The Un-Sets, imo, are really something that ought to be kept around as they offer freedom, creativity and forward/very forward thinking