And I also hear from some people that the French Horn is sadisticly hard
Yes, if you've ever played French Horn, you would know how hard it is to hit the right notes. Even after you learn a piece very well, you can still mess it up.
Mozart's Concert Rondo (K. 371) is tough (for horn), but Mendelssohn's Midsummer Night's Dream has killer high notes, and you have two transpose half a step, but at least its slow. Haydn did some painfully slow and high pieces that just kill your lip after a while (but they sound great). I'm sure there are other horn pieces harder than this, I just can't think of any.