I don’t know of any comparisons, but anecdotally, I play both piano and button accordion and my guess is that piano keys would be easier for beginners, due to the direct mapping to sheet music. Beginners start in C major using just the white keys and then added sharps or flats are the black keys. (Then you learn to play in other keys, but gradually.)
A chromatic button accordion has other compensating advantages but I don’t find either one easier overall. They’re just different. You get better with practice.
Piano teachers today sometimes start in other keys that are easier to play scales in than C Major like B major and D-flat major! And this way kids aren't "scared" of the black keys--they start with them.
A chromatic button accordion has other compensating advantages but I don’t find either one easier overall. They’re just different. You get better with practice.