On the violin, the Chaconne is particularly demanding because of its long length, complex chords, and intricate fingering. Violinists must have a strong left-hand technique to execute the fast passages and precise intonation. They must also have a good sense of rhythm and phrasing to bring out the music's beauty and expressiveness.
On the guitar, the Chaconne is challenging because of its complex harmonies, demanding fingerpicking patterns, and need for perfect timing. Guitarists must have a solid right-hand technique to produce the clear and even sound required for the piece. They must also have a deep understanding of music theory to interpret the harmonies and create a musically satisfying performance.
Ultimately, the "hardest" instrument for the Chaconne depends on the individual musician's skills and strengths. Some violinists may find the Chaconne less challenging than some guitarists, and vice versa. However, both instruments demand a high level of musicianship to perform the piece well.