IMO, the piano is far more complex to play and difficult to play well than the organ (or harpsichord).
The weighting & balance of notes in chords & textures, the precise colouring & nuances (achieved purely by aforementioned weighting and articulation and use of all the pedals - singly or together, including half pedal and flutter pedalling) and the advanced effects achievable on it (different kinds of staccato, ghost notes by depressing keys silently while playing others, even stroking the strings while depressing some keys etc, etc) are all impossible on an organ. On a piano, you need to have perfect control of each individual finger in order to get the sound quality you want. On an organ, you just need good finger independence.
That's why pianists transition easily to organ but not vice versa. In fact, most organists were once pianists, but rapidly lose their pianistic skills if they don't return to the piano regularly.
I played the organ at school occasionally during services or choir practice (- never having had any lessons on it), when the organ scholar was indisposed. He never really played the piano, and whenever he tried to play it, it sounded really rough - uneven voicing, ghost notes, other notes sticking out. Yet on the organ, he sounded like a professional, and his foot technique was of course better than mine. We once had fun challenging each other to play Bach's Toccata & Fugue in D minor (BWV 565) on both instruments......my rendition (with a few necessary transpositions) on the piano was much, much better than his on piano, whereas our respective organ renditions were more closely matched. (We both agreed on the results
A lot of an organist's work is done beforehand - deciding on the stops and the keyboard to use, and (especially if an unfamiliar organ) knowing exactly where the stops are.
BTW, someone mentioned finger switching on organs - advanced pianists use it all the time on the piano too. Sloppy pedalling or using the pedal when finger switching to achieve desired legato should have been used is rampant amongst lazy amateurs (or those who haven't mastered that art)......