I'd say that creative subjects like music or art can be a lot harder because ultimately you can only get to a point before practice doesn't help anymore. You can argue eh same for History or Maths or Chemistry as you have to be a good writer or exceptionally perspective or just understand maths and sure thats true but someone with a natural ability in maths who doesn't practice can do a lot worse then someone who practices everyday , whereas in music or art, you may practice technique everyday and still draw utter crap compared to someone who messes about and then produces a masterpiece. But it can also be strongly argued that academic subjects challenge you in that regard, you may be good at them but you have to learn to be even better whereas if you pick a creative subject, its probably in your comfort zone, so in that regard Academic subjects are a lot harder.
Too long; Didn't read it depends either way and what your criteria for challenging or easy is.