I don't really do unpaid collabs anymore, but I did several in the past and learned a lot from them via trial and error.
When an artist is willing to do collabs, its often because they're interested in an opportunity to grow. Either they want to take on an unfamiliar genre, or just get some practice. Another reason is sometimes you just want to do a fun casual project on the side because maybe you're getting bored or frustrated with your own stuff.
In these cases, you would want to partner up with someone that actually offers more to the table, someone you can actually learn from or is able to do both writing and the art. You would also want it to be short and sweet like others have said. But unfortunately this is not what a lot of these collab posts offer. Rather, you get a lot of long difficult projects with possible revenue down the line and 50/50 split.
So to give an example:
I sincerely tried to do the work as an artist partnering with a writer working on this long fantasy epic webtoon hoping it could eventually become featured on Webtoon. And the writer really tried as well, they tried to help with layout and lettering, wrote a good script that came with references, and communicated well (we even video chatted . . .). I tried to make that connection and was determined. . . . but I eventually burnt out.
I even made a thread about splitting future revenue because even though I really wanted to, it was really hard to believe that with me doing all the art it would be 50/50 if we were being "fair" and even if you look through that thread I'm so agreeable and optimistic it actually saddens me cause now I know there's more to say about the topic. We got a a bit of it off the ground but eventually I had to tell the person I just can't put in this kind of work for free blindly hoping Webtoons would notice the project quickly enough, not with everything else going on in my life.
After that any collab that I did was super super casual or a short time investment. Also, I required that all of the writing was finished since my main focus was on the getting better at art and webtoons. Then it got to the point where I was able to do comics professionally and now unless I really really want to, unpaid collabs are just not on the table.