I thought the same till I tried a full size one - they are much smoother and easier to get a tone out. But like with any instrument, these virtuosos make it look way easier than it is. Hell there are accordions that are button-only on both sides, no piano keys. Those button matrices have redundant/repeated notes on them so you don't have to overextend your fingers to hit the notes you need, but it sure makes for a head-scratcher when you try it for the first time.
been listening to Novos Baianos but haven't seen that doc