Come on this one is simple the hardest instrument to play are string instruments but any instrument is hard at first but once you have the basic understanding of music it is always easy to pick up a second or third instrument but in my experience the stringed instruments are the hardest because they require the most technique as to how to hold the bow , bow placement on the strings, finger positions and the list goes on and on you can get away with sloppy technique on some instruments but you have to have proper technique on stringed instruments therefore in my opinion they are the hardest to master.
P.S You also have to have a very good ear because there are no frets you can place you finger in between a f# and G unlike the paino when you hit a note you get that note on a stringed instrument if you have a G# you can play and A flat and move you finger a hair back and get a true A flat.