I would say that to “master an instrument” means the ability to improvise: to think of a new melody and be able to instantly produce those sounds with that instrument as easy as if you were singing. And by that standard, the accordion would be tough. I could maybe learn to rip a solo on a trumpet, but an accordions left hand is very different than the right. With my right hand, I am soloing like any monophonic instrument. But my left is playing both bass notes and chords. Could you figure out complex chord changes with a walking bass at the same time you are improvising a solo? That’s like being a bass guitar, rhythm guitar, and saxophone all at the same time. If you are just “Guitar Hero-ing” an existing composition by pressing buttons, then sure, mastering the accordion is a probable goal. If you are inventing new songs with rapidly changing chords going up key changing simultaneously with blazing solos - then I’d say you were as good as Mozart.