Lee, I started my musical journey at the age of 13, learning to play 'piano' accordion ... the studio I studied at (and later taught at) had soloists and accordion orchestras - like John C - and we played all classical pieces, Rossini, Tchaikovsky, Beethoven, etc. in competitions and concerts, including Carnegie Hall in NY. I don't know that the accordion is any more difficult than any other instrument, because, like other instruments, it is learned one step at a time ... also, the left hand is laid out very logically in the 'Circle of Fifths' ...
To ME, the chromatic button accordion LOOKS more difficult than the piano accordion, but looking at some diagrams, the right hand seems very compact and gives the ability to perform runs utilizing very limited movement over the keyboard.

PS: I have found playing two hands separately on a piano quite difficult, and I marvel at organists and drummers who can have feet and hands doing different things at the same time ...