Eternity is not broken because the AI doesn't do well with predicting the future.
If it had an Eternity in hand and waited to play and then use it against your creatures when you had a low count, and stored up reverse times to deal with you flooding it, you'd find yourself in a lot more trouble.
Edit: Also a thought about true pvp. If there's any concern about hacking decks or other issues like that, initially I think it would be awesome even with no reward. So you'd select to play, enter the name (Match up via forum or something) and then have the pvp match when your opponent did the same for you. No coins, no cards, no reward at all for it -- just the fun of getting to test your deck against your opponent. That seems like a good starting point.

