I made a deck called 'The coin toss' it basically shows the unfair ness of some cards, with 5 hp if you can put a creature out on turn 1 you win, else Silence will silence you for the rest of the game. With 900 hp it would be unbeatable, just putting it out there that Silence is really way to powerful, especially saying you can have it in like a 15 card deck.
Players may only have 6 of a given card in a deck...
So, in regular PvP, silence is not imbalanced.
With 900 hp, there would be a lot of decks that would be virtually unbeatable. But, you shouldn't judge a card's balance based on the NPC challenge decks that can be created with it.
NPCs that are difficult or even nearly impossilbe to beat are not necessarily a bad thing. It gives players a way to create and submit challenge ideas for others to try and overcome. As long as decks like that are used in special situations (e.g. questlines, world events, easter eggs) they can be quite fun.
It all comes down to how all these submitted NPC ideas end up getting used and distributed once v1.4 is released.
Arena decks could be a problem, however, if they are allowed to ramp up to 3x draw and over 200 HPs. In that case, silence would be badly imbalanced and could disrupt the arena meta-game pretty badly. I think that line of discussion may be better suited to other areas of the forum (card balance or arena related boards).