Not when it's RNG and goes from weak to game winning.
Without variance, this game would suffer. If everything was static it would be boring. That's what the game is about, doing something based on what the other player does on his side of the board. Depending on what the player does, some cards may become more powerful than their default state. IMO that is a good thing.
Viper Witchers has this kind of mechanic, but even if it banish a gold card, it's still not a great move, because the opponent have more than 1 gold card, and should not base his whole tactic on a single card and/or a single play.
A forfeit is a win and there are plenty of decks built around one or two cards. No player should be able to strip their cards right out of their deck. Whether it's RNG based or not.
Well, you didn't play out the round, so the Viper Witcher did not win the game, the forefeit did. That's down the the opponent, there are many reasons to forfeit. You also win if the opponent have to go to the toilet and forfeit to do so. It's not a win because of anything you did. Same with banishing a good card from their deck. They could play on, but they did not. You did not win because of the game, you won due to the opponent forfeiting. It wasn't due to the Viper Witcher, it was due to the opponent being a weak player with a weak tactic who gave up way too easily.
It's a different tactic which happens to also be bad design.
Why do you think it is bad design? I happen to think reveal and vipers is a good design, enriching the game with more ways to play and more tactics. Vipers are useless in most tactics, but with some tactics they are core cards.
There are plenty of interactions with the other players cards (including Graveyard), and it certainly is not only with Vipers or Nilfgaard. I think that has its place in the game, and the game would be less good without it. In regards to Nilfgaard, this is one of their specialities and it would be a bad thing to remove it. I think reveal should become better and more purposeful, lie revealing low cards and high cards and such things, so that people playing with reveal decks, can gather information about the cards of the opponent, at a reasonable cost ofcourse. With Vipers, at the very worst, you thin the opponents decks by 2 cards they don't really want or need. At best you can duplicate Vipers and say banish 3-4 gold cards, in most cases, the "ideal" scenario is that you banish 2 gold cards. Very few people play Vipers to play more than 2. It's fully possible to win a game after losing 2 gold cards.
Reveal was absolutely horrible in homecoming. I find it strange that they threw out so much in old Gwent that worked pretty well and instead put in a bunch of RNG style nonsense.
RNG? Random number generator? aka "random"?
I think randomness belongs in card games, but that's just me. Imagine how boring it would be to play card games (example poker) with an ordered and unshuffled deck.