There's no granularity, though. Its either "The DM has decided it was difficult enough to level up" or "it provided no numerical growth to your character." No in-betweens, especially on the player's side.
For example, if a player quits halfway through the quest they started, they may think they deserve at least partial exp to level up but if they get nothing, it will feel like a waste of time that they even pursued it in the first place.
Essentially, the players get to pick what they choose but they get punished for finding it uninteresting later on.