I wondered at first if this should not work on Rares, but thinking it over, it's such a niche effect that you would have it for the very purpose of discarding a Rare full hand replacing card like Atlantis. It's not overpowered because all that the opponent needs to do is keep a Common summoner on hand such as Golem messenger, Book of life, Draft, Watchtower, Lady Ellena... it's quite a long list.
In fact, I'm starting to wonder whether it's likely you'd EVER get to discard anything better than a Common. Even my tower building deck has a Common with a summoning effect. It looks like my tower attack deck would have something to fear from this, however, as the only affected cards are Rare (Thor and Architectural improvement).