I have written on the piano but I "think" way faster on guitar. Plus, I can improvise toward compositional ends on guitar in a way I can't on piano. But, for a different musical perspective it can be really gratifying to change things up.
I had to write an entirely composed concert piece for solo piano and forced myself to get 75% of the way through it using only the piano, or extrapolating ideas out on paper and then checking on the piano...the last 25% of problem-solving was worked out on the guitar even though it couldn't technically be played on the guitar.
I can't remember the whole story but when Joaquín Rodrigo wrote Concierto de Aranjuez he did get consultation on the guitar parts.
Rodrigo played piano, but the piece involves guitar.
I hear that... I'm a gtr player and not a keyboard player. The serious work on keys has to have a "real" player that can ring a bell with keys.. I can do basic ( which "sounds" like I play well lol )... but flying with keys and doing what can be done with them is for a real player... Like the way I can play gtr. I like collaboration in composing/writing music... a brilliant keyboard player can "think" keys and do what I might "hear".. but I certainly can't turn on a dime with it.. let alone really play most of what can be done with it. I can do that with a gtr.