I agree with the balance in difficulty, I think they did really well with Generations, as the later levels were pretty challenging, though they did usually hand out S Ranking like candy, that they should have fixed a bit, but in said later levels I had to work for it. For 3D games I'd say SA1, SA2, and Generations have good difficulty balance, been awhile sence I played Heroes, and I have yet to play Unleashed so I can't comment on them in terms of difficulty.
As for 2D games I have yet to play any episode of Sonic 4 so no comment on them, though Episode 1 looks pretty easy, but since those are the first levels of Sonic 4 as a whole that should be obvious. The Classics (1 - 3K and CD) are good examples of difficulty balance, it can be easy to finish with practice, but hard to master if you want the best ending, now for DIMPS, that's a bit tricky to explain. Pocket Adventure had a good balance, though it can be more challenging then other games I found that a good quality in the game, everything was handled well. The Advance games, despite being good games, have some drawbacks, mainly reaching the Special Stages, they made that WAY too hard in those games, Advance 1 I can bare as they were always in the same place, but for 2 and 3 you have to go a certain way or you can go to those stages, 3 doesn't have that problem TOO bad, but the hunting was a little too much in that game. As for the gameplay overall, they did it well. I didn't see a problem with pits in 1 or 2 though 3 did have a few of them. Overall they did good, as for Rush they did push pits a little too much, not so bad in the first game but Rush Adventure did have more pits then they should have at some areas, as for the special stages they fixed that problem Advance had, much to my enjoyment. Layout wise they can be weak and got a little weaker over time overall, but for difficulty they weren't that bad, just in those problem areas.
I can't comment on Generations 3DS but that's my view on the difficulty on both 3D and 2D games, if they get rid of more of the infamous pits they have increased over the years (which they have when it comes to Generations) and not have Special Stages being nigh impossible to reach then they would have a good thing going in the future.
EDIT: I'll say this though, in Generations the bosses died too quickly, they should have at least 8 hits to kill them.
This post has been edited by Mike Arcade: 22 May 2012 - 02:29 PM