Yeah! I remember back to a thread a while ago where a colourblind writer was asking if his artist had indeed changed the hair colour of a character because his audience mentioned it. I think it was brown to pink. Small things like that even can totally destroy the mood/ feel/ overall aim of a story, especially if it was supposed to aim for some level of realism. (Also designs changing midway without explanation obviously being jarring for the audience.) Collaborations really need to be approached with a good level of respect for the other person's respective field and area of expertise so things like this don't happen. It's an absolute must that you don't end up fighting over all these petty details, and I think most importantly you need to know how to back up your position if you want to assert you know something would be better a different way.
I've fought for a few things in my favour artistically but it only works when you can bring a solid reasoning behind it, not just "but it looks better my way!". If I see some designs being a certain way it's because I'm trying to work off of the basis the writer gave me for the story rather than just insert whatever I feel like. And usually it ends up in a compromise where get half of what we want out of it. Let's use a random example like say we're doing world building and we know we need to introduce a bunch of fantasy creatures based on unique environmental circumstances. The audience needs to know it's not earth.
Artist: "I have this idea of a bright orange bird that might have disproportionate features, it's head looks like it's too big for it's body, and probably aquatic."
Writer: "Well it would need some kind of obvious anatomical mechanism to balance it's large head so it doesn't just topple over when it stands upright. It also likely would have webbed toes rather than the standard zygodactyl feet of perching birds you drew. And why bright orange, underwater? What's the evolutionary reason for it?"
It's not the best example I can give but that general idea should be part of the design process, haha. Which I guess brings me to essentials both people require:
Assertiveness. Passive or aggressive (or WORSE: passive-aggressive) behaviours are red flags. Your partner should be able to tell you in all honesty when they do or don't like your idea avoiding passive terms and without calling you names. If their way of communicating it is just changing things without your consent, run.
Ability to take criticism. Someone who can't accept hearing why their idea is bad is not suited to this environment.
Keep in mind you're not destined to be stuck with these traits forever, either. Someone might have trouble taking criticism but use teamwork to eventually overcome it. However, lack of both will make it a painful and unsuccessful process. When looking at your potential partner's work, maybe see if you can find any feedback they have been given in the past and how they handled it.
It could be expanded a lot more to look at what the artist should be expected of by the writer. I don't want people to get this idea it's this power play where the artist has all the leverage. (And therefore may look off-putting to writers who assume it's largely an attack on them.) xD