It´s their first iteration, if we see more in the future they might be more balanced.
I like the quest concept. I wish they would have made smaller epic quests you didn't have to build your deck around.
I somewhat disagree. Dirty Rat makes the game more complex, which I find to be a positive (yes, yes I know that the game is meant to be very fast paced & easily accessible, but still). The reason I say Dirty Rat makes the game complex is because it adds more avenues of interaction that you can more or less force from your opponent (in this case it is the ability to affect crucial cards being hoarded by the opponent in their hand). Take a look at Yu-Gi-Oh for an example, back when I was still actively playing it you had cards like Yata-Garasu and Delinquent Duo, both of which heavily influenced the creation of deck strategies or entire archtypes that could be viable win conditions to shut down your opponent instead of merely waiting for them to pull of some combo to smash your face in. From the way I see it Dirty Rat forces the game to not feel as 'lazy', where you safely draw into your threats, your combo pieces, your big tempo swing minions, and coast to victory with no worry so long as you draw at least somewhat decently and stall long enough.
As for the armorsmith I have to ask, "Why?". I would never count on the card to be nerfed. Belcher was a much better taunt body with the spawned ooze and not once was it touched. Justicar was an infinitely better armor generation machine and surprisingly it also was never nerfed for warriors. Armorsmith is a mediocre and pale shadow of the two of those cards combined. *Shrugs*
They still easily could in the future. However, they would have to make non-legendary quests restricted to one per deck also, and they would also have to figure out how having more than one type of quest in a single deck would work exactly with being added automatically to your hand.