I played all the levels on Easy, to see how far you can get with that limited # of stars earned, and had trouble on a few of the modes as I learned them (especially elimination). The regular races have been cake once I learned stunt boosting, and playing them again on Medium hasn't been too taxing.
I played Hard on the demo, and it pummeled me, so I'm curious how I'll do once I step up to it with a new skill set and leveled up characters.