It will always be broken as it is because it's essentially a no-risk, "play with 11 cards whil your opponent only has 10" card. You have absolutely no reason to not include it in your deck. It either needs to have high attack (especially as it won't be affected by weather then) or not be a hero (though then it'd be very close in power to Thaler). Or some other drawback somewhere.
But I've had that argument on reddit as well and found my opinion in the minority so whatever, I guess something's just wrong with me.
It's not broken at all. I think it's a great addition to the collection. There are a ton of spy cards. There is also a leader power that lets you start with one more card than your opponent. Is that broken? No. Of course not. And it is very, very far from "play with 11 cards while your opponent only has 10" card. Your opponent can play other spy cards. Your opponent can play THAT card. Then, provided they had 10 in their hand when they played it, they will also have 11 cards. I mean, this isn't rocket science. Thaler is almost as good as the "Mysterious Elf" and he's simply for sale in an inn.
Fact is, I used to haaaate when an opponent played
It usually meant I was in for a whooping, because his deck was so much better than mine - especially if it was N. Realms and had a few decoys. We'd go back and forth, back and forth, spying and decoying. Now that I have that mod installed, though, I can not even bother with spies in some decks and beat the players who throw Avallac'h out in the first round.
Also, there are other hero cards I would put in a deck WAAAAY before
. Geralt and Ciri, for one. Are they broken because they're 15 points? I throw them down and I now have 15 points while my opponent has 0. There is absolutely no reason not to include them in a deck. One of the reasons they're hero cards.
What about Yennefer? Is she broken because I can use her to play my opponent's Thaler and have one more card in my hand afterward? You see where I'm going with this?
So, to sum up: Not broken.
PS. If you hate it that much, why not make a simple mod altering its stats? Boom - your problem is solved.