And I'm only talking about lead guitar here because that seems to be the one thing people talk about when they complain about this. I think there is a TON of challenge in playing all instruments through on expert, and then ever more obviously with singing and playing guitar on expert.
But in regards to lead only (on expert), I'm having a hell of a time 5*'ing the majority of songs in the game. I've got 5*'s on 15/45 songs so far, and none above the "solid" tier. The scoring is insanely tight; I feel like if I miss 5 notes in the song I might as well start over. And while there are obviously no massively insane guitar solos in the game, the overall charting is MUCH better than most any other games I've played. I just find these songs much more enjoyable in that they're actual structured songs rather than a lot of the stuff at the highest tiers in Rock Band which are generally just the craziest guitar riffs they can find for the sake of difficulty.
I'm sure there are people here who think I'm horrible at the game because I haven't 5*'d every song in the game already and have multiple FC's, but all I can say from a "mainstream" user point of view is that this game is PLENTY hard enough for the overwhelming majority of the audience. I know a lot of you on here are really, really good, but for an average Joe like me I see plenty of challenge here.