i havent done unpaid collabs, but i did a few drawings for a friend tryna make it in music when i was starting out - for the second lot he asked of me he said he could pay £50... he ended up fucking me around with what he wanted, and when i told him i wouldnt continue until i got half the money upfront (after already providing him w a pair of half decent ep covers) he said 'okay' and we didnt talk again. i could tell all the way through though that 1: he had no idea what he was doing when it came to album art, and 2: he didnt appreciate the work i was doing. this wasnt bc he was a bad person necessarily, just someone who didnt understand the craft. its really important to work with someone who understands what it is youre doing for them and its value
in terms of unpaid comics collaborations, i think ive made my views p clear over time - itll work only if the collaborators come in on the ground floor as equals, their input, limitations, and artistic development considered in a project theyre equally invested in, with equal ownership. these kinds of collaborations require personal chemistry - take Good Omens as an example (not a comic, i know). gaiman and pratchett met for an interview, and hit it off - then pratchett offered to buy gaimans idea for the book to write himself, and they ended up collaborating on it due to mutual passion for the project and good personal chemistry.
as a writerartist myself, i also feel that if an artist or a writer wants to get started in comics, they should just go ahead and learn to draw / write enough to make a serviceable comic. imo, if you wanna work in comics, you need to have both an understanding and appreciation for visual language, a good eye for design, and an understanding of narrative - and it dont need to be that hard.
i dont mean that writers whove never picked up a paintbrush need to buy a cintiq and study anatomy for a month - really simple, even 'ugly' art can be not only serviceable but even compelling. it can be collage, it can be glorified stick figures, it can be 100% in biro pen. just do it - if the writing itself is good, you can use it to convince more experienced artists to take you up on future projects. or hey, you might just get the drawing bug, and from your raw perspective make something even more genuine and original than a self-proclaimed artist could.
similarly, artists dont need to study hemingway or whatever bollocks to write decent comics - God knows most of these ppl asking for your unpaid labour havent. if you like comics, you like stories, so you can study those stories and figure out what they do. every movie you have ever loved has 50 videos on youtube explaining why you love it. anyone can tell stories, telling good ones can be learned through studying structure, the stories you love, and real life. cant come up with ideas? random idea generators, fanfiction, AUs, one of those weird fantasies you have in your head, it is good enough, trust me. get it down on paper. do it! you can!
neither writing nor art is some kind of inherent lofty skill that you either have or dont, its all learnt, and if youre working in comics and you dont have an understanding of narrative or of the fundamentals of art and design, youre at an unnecessary disadvantage. stop fishing for someone to do it for you, get your hands dirty and learn.