1) Personally, I love this new mechanic. It changes how you play the game. For example, there was a post in the FFG forums where it was said that mobility is the most important characteristic of a hero, but as I said on my post https://community.fantasyflightgames.com/topic/226760-is-mob...
because of the tainted cards (and other things) now survivability is more important. Another thing is, I am a very aggressive Overlord, so my play style almost tend to be "do not take prisoners", even when the quest is not meant for that. Therefore, this cards (and quest from those campaigns) align with my play style and let me enjoy the game as I want hehehe
2) Although I would love to use them in other quest/campaigns, I think it may break the balance and the structure of the game. Because of what I said about survavility over other characteristic, heroes like Orkwell or Rendiel would then not be so interesting to use.
3) I think it will depend on how strong the effect would be, because for example, if you put a condition, no one will chose it and you will get stuck with the same cards over and over. Another thing can be that by choosing the effects, the power of the card can be lost. A hero will create a strategy in order to minimize the negative effect and will be always prepare...losing that "challenge" that comes from the surprises.