thebitterfig is right. To add to that, It is also worth noting that a huge part of Baku and Genn's balance issue was they always gave you the hero power buff. Zeph is not always going to be in your hand, nor will he help you if you misplayed, He does require a bit of skill checking: when to play him, guessing and setting up possibilities for the best outcome, asking if it is worth torching the one off get out of a pinch situation card now or later, should he be saved to finish the game, etc.
I think Zephs swing potential is astronomical, there is no denying, but it does not make him a terrible card, just a very powerful one, which is what you would want in a deck full of one off cards. I would go so far as to say DQ alex might need a revist to give each of her dragons 1 mana because the swings off her can basically end the game. Zeph cant do that unless you were about to lose anyway.