The French horn is widely considered to be the most difficult brass instrument to play.
The modern-day double French horn is actually a combination of two horns (F, Bb) that share a common mouthpipe and bell.
Because the partials (available notes in the harmonic series) are so close together in the third octave, the horn’s most comfortable range, it is fairly easy to miss or “crack” a note. Conversely, the partials that occur in the trumpet’s most comfortable range are comparatively more spread out, allowing for a bit better accuracy and more control.
Another reason for the horn being difficult to play lies in the fact that it has the widest range of all the brass instruments: nearly as high as a trumpet and almost as low as a tuba. Additionally, the horn mouthpiece is small and deep, requiring fine muscle control and embouchure development. The mouthpipe is long and slender, creating more resistance for the player to overcome.
Because the horn faces backwards–as opposed to the other wind instruments–the horn player must work extra hard to balance and blend with the other members of the wind instrument family.
Excerpts from https://houghtonhorns.com/why-is-the-french-horn-so-hard-to-...
The French horn is widely considered to be the most difficult brass instrument to play.
The modern-day double French horn is actually a combination of two horns (F, Bb) that share a common mouthpipe and bell.
Because the partials (available notes in the harmonic series) are so close together in the third octave, the horn’s most comfortable range, it is fairly easy to miss or “crack” a note. Conversely, the partials that occur in the trumpet’s most comfortable range are comparatively more spread out, allowing for a bit better accuracy and more control.
Another reason for the horn being difficult to play lies in the fact that it has the widest range of all the brass instruments: nearly as high as a trumpet and almost as low as a tuba. Additionally, the horn mouthpiece is small and deep, requiring fine muscle control and embouchure development. The mouthpipe is long and slender, creating more resistance for the player to overcome.
Because the horn faces backwards–as opposed to the other wind instruments–the horn player must work extra hard to balance and blend with the other members of the wind instrument family.