I struggle through the final 2 chapters of Great Marble Adventure's campaign (well, at least the default versions). These already feel hard enough for me. By this point, I have a good idea of how this game likes to operate.
The physics are nice and noticably tighter compared to MBG. The levels are generally much longer and complex, but their labyrinthan appearance gives way to well placed markers and a wealth of direction hints so you never lose track of where you're supposed to go.
That being said, the levels in general don't offer too much in the way of structure. Most are comprised of a dozen sections of platforming before you reach the end. I would have liked to seen some more gimmick or one-off designs like MBG had scattered around. But that's perfectly fine since the game offers heaps of play time for your money (and those aforementioned levels are probably in the extra section anyway).
Ah yes, the 'extra' chapters that I flicked though towards the end. I thought it was over. But I was very, very wrong. Looks like I'll be going though those at some point in the future as well. But for now, have a good one!