I see these three cards being called annoying, frustrating, bad for the game etc. so often that I think it's time I commented on them in more detail and depth.
First off, I have been playing Usurper since October, and I have always kept two Viper Witchers in my deck, so yes I am biased when it comes to those cards. I will, however, explain my reasoning and opinions in as much detail as I can.
Traheaern I tried out once, but he was too inconsistent and so I replaced him with something that actually synergized with the core of my deck.
What follows is my personal opinion, and I do use a fair bit of imagination to explain my views.
Usurper
Yes, he disables the opponent's Leader, but that is literally all he does; he has no actual ability, and he provides very little extra provisions in deckbuilding. If the opponent's strategy is not dependent on their Leader ability, Usurper is not much of an issue and can in fact be inferior to the other Nilfgaard Leaders.
If a deck is entirely dependent on its Leader, then Usurper will completely ruin the strategy, sure. But is it ever a good idea to have a deck (an army) that cannot manage without its Leader (general)? Leaders (generals) can get disabled (killed, injured, or captured), but the battle can still be won by either side as long as they have a (back-up) plan and are smart/strategic with their moves. Usurper only eliminates one, although admittedly major, aspect of the opponent's strategy. He does not touch the deck itself, not by himself.
And yes, I have played against Usurper as well, with Leaders who have actual abilities. Yes, it is annoying to not have my Leader ability available, but it is not the only annoying scenario when it comes to opponent Leaders.
Lastly, about Usurper's ability as a game mechanic. Nilfgaard is a faction that likes to mess with opponents' strategies: stealing units, gaining boosts from stealing units, spying on opponents' decks (Reveal), infiltrating enemy ranks (spies, Infiltrator). In this light, Usurper's ability makes sense. He is a Leader, so he should have a stronger ability than non-Leader cards. Keeping in mind the game is, at its core, designed to be a battle between two armies, Usurper's ability translates to incapacitating the enemy general. Maybe he slips them non-lethal poison, maybe he has them captured on false charges, maybe he distracts them or lures them away from the field of battle. Usurper is not a fighter, but he is cunning and determined to gain an advantage using less than fair methods. He is, as his very name says, a usurper; he does not play fair.
Viper Witchers
They can be extremely good, or they can be extremely bad and even help the opponent by hitting filler cards, or they can be "meh". It all depends on RNG, which automatically makes Viper Witchers potentially be worth far less than their provision cost. In my opinion, that by itself means the card is not "OP" or even "auto-include". Which in turn means they are not an optimal choice for a deck, unless of course the deck's purpose is to mess with the opponent as much as possible. That is one of the purposes of my deck (the other being witcher synergy), and so Viper Witchers have never left my deck despite the fact they sometimes hit low-value Bronzes like Wolf Pack.
I won't deny it's satisfying to see a key Gold get Banished, and I won't deny it's annoying to see one of my key Golds get Banished if the opponent plays Viper Witchers and is lucky with their RNG. But all things considered, I like the current Viper Witchers more than the Alchemy ones in beta. *grimaces at the memory*
About their ability: it, just like Usurper's, fits Nilfgaard. I see Viper Witchers as assassins; they strike out of nowhere, and kill their target instantly. They don't shoot arrows, which would be the Scoia'tael way, but instead use stealth to get close to their targets for a melee attack. They are witchers, so they know how to use a blade effectively (Banish rather than discard).
Traheaern var Vdyffir
Like I already said, I don't consider him consistent enough to have in my deck. He looks at the top three cards in the opponent's deck, which is an RNG ability just like Viper Witchers'. Sure, you can see a key card among those three, but they can also be low-value Bronzes that would not do much for the opponent anyway. Plus, because Traheaern puts the selected card in the graveyard, the card can be revived later (unless of course it is Doomed). In fact, if the opponent has a bricked revival in their hand, Traheaern can actually be helpful to them and bad for his player.
As for his ability: this one is trickier. He is not a fighter, but an emissary with orders and a soldier escort. His ability could translate to him being attacked by a group of enemy soldiers, and his soldiers eliminating the group's leader (the card that gets discarded) thus stopping the attack.
(I did say I was going to use a fair bit of imagination!)
----------------------
If you've read this far, regardless of what you think of what I wrote, thank you for taking the time. I had a good time writing this post, putting into words what I have been thinking for a long time now.