Something doesn't have to be an automatic win card to be overpowered. It just needs to unbalance the game--make it so that players' in-game tactics and book-making decisions are significantly altered to compensate for it. If nothing else, the fact that a great playtester like you used such an obviously pathetic argument proves that you're clutching at straws.
I am of the strong opinion that the metagame of EVERY customizable strategy game is an endless experiment, an eternal balancing act. There are so many complex factors, so many different ways to play. And the people who work so hard to make such games work are only human. No matter how hard they try, some things will slip under the radar, they WILL make mistakes, and there WILL be misprints.
That being said, I think that its possible to minimize how many mistakes are made, and then directly fixing the ones that are made, rather than trying to compensate for a mistake with more mistakes.
I think the reason card edits after printing look so bad to people is because people are evaluating this game in the same way they would evaluate games from other gaming genres. I personally think rulings to fix errata are actually vital every once in a while in a customizable strategy game.
What are you going to do when you start releasing different formats (including multiplayer formats)? Cards that are well balanced in one format might sometimes be game breaking in another.
I also strongly suggest that each format eventually develop a banned/restricted list on an as needed basis. Maybe you could create a sort of Epic #2 trait, where a maximum of two copies of a specific card with the trait can be included in the spellbook.
Please consider what I have said. It's not too late to fix things.
