At the beginning I remember me and my girlfriend struggling a lot to get through the core set, then we bought a second and a third copy, and everything went better (we still lost a lot though, especially in Dol Guldur).
With the APs and expansions everything got a bit easier for the core set, but not for the packs (I find return to mirkwood still one of the hardest scenarios, hard core if you play solo without a spirit based deck). Khazad-dum and Dwarrowdelf improved the card pool while not really changing the game (so the previos cycle got a bit easier while some of the Dwarrowdelf quests were a bit tough). Now we have HoN that has changed the game completely. I used to do a whole playthrough with a deck (swapping only a few cards to adjust my deck for each scenario) but now this is impossible if you build a "tacticsless" deck, because in Hon you need it (at least to win consistently).
So yes, the game has changed, and yes, sometimes I (we when I play with my girlfriend), struggle but I guess it depends on how one approaches the game. Most of the times I try to build funny and thematic decks (I seldom build all-winners because I find them boring). All in all I think the game is scaling pretty well, old scenarios are easier (some a lot, some not very much), but most of them are still fun to play and can still beat you.
My conclusion is that if you build a different deck for every scenario, then yes, they're much easier, but if you play with one deck, trying to improve the theme or the strenght of the deck then only some of them are easier, some other may even be more difficult (for exemple playing shadows of mirkwood with a rohan deck is much easier than playing it with a gondor deck).
Some specific cards are helpful everywhere or almost (Hands upon the Bow) some other cards might be very helpful in a few scenarios but wasted slots in other scenarios (Ever my Heart Rises), and others can be game changers in many situations but still pretty useles in others (like Erebor Battle Master, awsome in Shadow and Flame but wasted in, say, the Hills of Emyn Muil or even the Dead Marshes or a Journey to Rhosgobel). Up to now I don't think there's been a card that, when released, has broken an older scenario. On the contrary it helped making that scenario more viable for spheres that were weaker in that specific scenario (like how Glorfindel made Spirit better at fighting, or Hands upon the Bow made Tactics better at questing, though not by adding willpower, or even how Elrond made elves playable)
As for how much I get to beat a scenario let's say that, with the average deck I build I get to win like 3 out of 3 when playing Hunt for Gollum, The Dead Marshes, The Hills of Emyn Muil and other easy scenarios like Road to Rivendell, 2/3 when playing those like Rhosgobel, The Watcher in the Water and so on (which are a bit harder in my opinion), 1/2 against tougher ones like the massing at osgiliath (even though lately the massing has become quite easy if your threat is low enough) and maybe 1/3 (sometimes 1/2) in the newer and harder scenarios…these data are purely to explain how I feel about scenarios and how consistently I get to beat them…but they may vary greatly though…because I even got a 3/3 in the siege of cair andros with an eagle/gondor deck once (at my best) or (at my worst) I got beaten hardly by easy-peasy scenarios when the encounter deck played tricks on me XD….The Hobbit scenarios are a bit different because deckbuilding for them is a bit frustrating at times (especially in scenario 3 of the first box and scenario 2 of the second box)…still I get to beat them quite easily, except for the battle of five armies which is tougher (and much much better then the other scenarios in the hobbit)
hope I didn't write too much XD