Up to the Shadow and Flame I had really believed that there is a deck that can win pretty much every scnario (Dol Guldur or Rhosgobel aside) on a very regular basis. I had also thought that minor tweaking, like the elsewhere suggested 5 card "sideboard" could take care of some specific quests (like Rhosgobel).
But right now I am happy to see that this is no longer the case. I mean I partly see it already but expect to see it much more clearly with Heirs of Númenor and Against the Shadow. The riddle mechanic in Dungeons Deep and Caverns Dim has partly thrown away the balance, as I feel decks with no uniformity of cost, type and sphere won't stand a chance. However, this type of quest could still be counted among those very unorthodox ones.
The siege and battle mechanic on the other hand, and I really hope it is a continuous approach by the designers, could not only give a lot of weight to the previously shafted Tactics sphere in solitary game play, but it could also disregard those Lore and Spirit high willpower low threat decks that used to run away from every quest up to the Hobbit or so. I have seen and played several of these and none of them had substantial attack or defence strength to deal with both the questing and the subsequent combat.
We are yet to see whether this supposition holds true or not but I have a hunch the designers have gone the right way with this new challenge to make this game, and the deckbuilding, much less predictable. For me, personally, it doesn't mean I shall switch to deckbuilding specifically for one quest alone but I shall try to have several decks ready, each hopefully with a different strength, and pick the most appropriate for each quest. Of course, this could have already been done up to now, as it is certainly no fun to have most of your heroes and cards sitting in the box, but for the Boromir side of character, it is hard to prefer the weaker version over the stronger.