Why is the French horn a most difficult instrument to play?

The French horn is a most difficult instrument to play for many reasons. First, the tone may be discouraging from beginning and inexperienced players. It's apt to be wobbly without dedicated practice and experience.

Second, a French horn's sound is effective only in the upper notes. It requires more effort to produce higher than lower notes. So a French horn player must breathe very hard into the mouthpiece.

Third, the French horn's mouthpiece is difficult to work with. It's unique in having a funnel shaped mouthpiece. All other brass instruments have cup shaped mouthpieces. So experience with most other brass instruments is no help in learning this particularly challenging brass instrument.

Fourth, the tube of the French horn is about 16 feet/5.3 meters long. That means a lot of length through which the player must project breath. And the tube is not only long, but big. In fact, it forms a big circle. The player's hands must be able to be placed around the edge of that circle. At the same time, the fingers must be able to touch keys that aren't user friendly. They're located in the middle of the instrument!