speaking from my experience playing the flute, piano is actually harder to play than forte, but faster usually is harder unless you're going rrrrrreeeeeeaaaaaallllllyyyyy slowly.
The use of dynamics to represent the difficulty of the levels is not based on the actual difficulty to play a certain dynamic on an instrument, since I agree that playing high quality melodies piano/pianissimo is really hard!! I play the violin, and playing soft with good tone is super challenging.
