What Makes The French Horn Notoriously Difficult?

The French horn’s register plays in a higher range of the harmonic series (essentially, the notes that are naturally playable without the use of valves), compared to other brass instruments. At higher pitches, it is incredibly sensitive to even small changes in the position of the mouth and air volume. That’s one of the main causes of so many blunders, and the reason brass players joke that the best way to make a trombone sound like a French horn is to stuff your hand into the bell and miss all your notes.