Bassoon. Oboe but bigger.Violin is easy to learn, but it’s harder to master. What are you considering the hardest instrument?
Oboes use lots of air. They only work half the time. Their reeds are priceless and also fragile as snowflakes.
Bassoons use even more air. They probably only work a quarter of the time. They have awkward reeds. I don't know how delicate they are but mhm they are pricey.
Here's the catch. It's not the instrument that doesn't work. It's the reeds. You might think they are broken. They may only work on one day of the week when the conditions are right and the stars align. *sarcasm
But still. Touchy things.