Seriously, when someone can be losing by more then 15 health then turn around and win because of one card, it is overpowered.
Having less health than your opponent doesn’t mean you are losing. Neither does having less board, or even cards in hand.
You are losing when you do not have response to your opponent plays (AOE if he has the board, way to fill the board if he just played an AOE, single target removal if he has a threatening minion, ect…)