The " usefulness " of an expansion is entirely subjective I think. You have to take a broader view of the cards included in the expansion AFTER it has been added to the main deck and even then after a number of plays.
The object of every expansion is not necessarily to make the game harder, like you might expect in a campaign from an RPG or a video game for instance, but to add a bit of narrative or substance to the game. Novel twists are added, like optional rules, or alternative endings and < insert cool idea here >.
The game ( story ) of Talisman relies on the deck of cards ( pages of its book ) which changes from game to game. You can travel around the countryside and not meet anything of note, except a lost Princess and a Wild Boar. However, the same journey on another day might have you facing a Giant, three Dragons and the Devil.
As The_Warlock rightly says, the difficulty of a card is determined by when it is encountered. We have all been surprised by the weakest of enemies once in a while.
When expansions are released there will of course be cards that people might feel are disappointing or " weak ". I myself would like to see a few " Über Enemies " but I am sure I wouldn't want to draw 3 or 4 of them in a row as that would spoil the game.
We all know there are " X amount " of expansions to come given the trend with FFG releases, and who knows what else besides. I am confident that enemy attributes and card numbers will all balance out in the end. Until then, I am prepared to enjoy the narrative that each game offers.
But V. can do that too so take no offense; he can be pretty astute too, and I don't think he's intending anything by sticking too his guns.