Personally, it's all about how hard the musical instrument is when you are a beginner, intermediate player, and advanced player. For example, the piano is one of the easiest instruments as a beginner, a so-so intermediate instrument, but a hard-as-heck advanced instrument because of the multiple note chords, complex voicing, and dynamics. On the other hand, an instrument like the violin is rather hard as a beginner because of the awkward technique, but definitely gets easier after that technique has been learned. That said, any instrument is hard to master, but some are harder than others. However, in my opinion, the organ is the hardest instrument because it is hard to play at a beginning, intermediate, and advanced level.
