Every instrument is difficult. Even the simplest of things like a frame drum or penny whistle can be difficult. But I think one of the most difficult to learn is the uilleann pipes. Liam O’Flynn, one of the great pipers, once said, “The old pipers used to say that it takes twenty-one years to make a piper: seven years of learning, seven years of practicing and seven years of playing.” It’s rather accurate.
This answer would vary from person to person, because it depends on what you would label “hard”.
For example, learning to play guitar is very hard a person’s fingers and will quite likely even make a person’s fingers bleed. All beginning guitar players get blisters, and then they get calluses. Some guitar chords (especially barre chords) can cause a lot of pain to the wrist. Therefore, this instrument is sometimes considered “hard to play”. By this definition, piano would seem really easy, because pianists simply press down keys painlessly. However, on the other hand, pianists have to learn a lot of complicated music theory, while most guitarists don’t learn any of that. Piano is usually much, much more intellectually challenging. You have to be smart to play piano, but not so much to play guitar. Because of the knowledge necessary to play the piano, it is sometimes considered “hard to play”. I have actually heard guitarists say, “I would never learn piano, because the music theory makes it too hard.” And I have heard pianists say they would never learn guitar because it’s too painful.
As you can see, it really does depend on the individual person to decide which instruments are hard. What’s hard for one person might be easy for the next, and vise versa.
In reference to other instruments, the double reed family(bassoon, contrabassoon, oboe, english horn) consists of some very difficult instruments. These instruments use what is called a double reed and often these reeds are constructed by the actual player. These instruments are found to be hard because all of the preporation required before even being able to play the instrument. The art of reed making takes years of practice and is a very delicate and tedious process. On top of that the reeds are very sensitive to the enviornment, often completly changing temperment with the altitude, humidity, and temperature. Once getting past all of the troubles of the reed, the actual instruments pose various unique difficulties. On the bassoon the left tumb is in charge of pressing 9 different keys, some of these just being one at a time or sometimes in combinations up to four. The right thumb is in charge of another four keys. Unlike any other wind instrument, the bassoon has keys that are not actually intended to be held down the full duration of the note. These keys are only flicked at the very beginning of the note just as the tongue touches the reed. The coordination for this is quite hard to master. The contrabassoon has a lot of the same technique reguirement of the bassoon just on a larger scale. Large hands are usually required for this instrument. A fact that remains true to all of the double reeds is the difficulty of tuning. With incorrect scraping of a reed the intonation of the instrument can be completly thrown off. This also holds true to the instruments quality of sound which is also greatly affected by the reed. On top of this the embachure (placemant and pressure applied by the mouth) plays a very crucial role to both of these aspects. Only after mastering all of these key points can one begin to effectively learn one of these instruments. The double reed instruments truely require much patience and skill to master.
As a pianist, I feel that the piano isn’t the hardest to play. It isn’t the hardest to learn or understand either. However, it is the mastery that is hardest. As for all instruments, mastery is the hardest. Comprehension and learning to play is different than learning to or rather having mastery of the instrument. I haven’t tried any other instruments, except for the guitar and piano. I do not have formal training in guitar, but I hope to learn to play it, though it seems kind of complicated with trying not to hurt yourself (lol). But I do not know about guitar. I THINK violin is hardest to learn to play though. Anyway, hope my answer helped.