So… I am one of those you mentioned. My primary focus for many years is/was UX/UI, but I am also an engineer and artist/animator, so I can and do build everything end-to-end. On most teams I have worked on in the last decade or so, the teams were large enough that there multiple people in a discipline. For example at Disney, there were usually at least 3 and up to 5 folks in UI. Usually a lead, UX, UI designer, UI artist. (sometimes split and/or shared duties depending on the skill mix). On those, I usually focused on implementation, vfx and prototyping. (but would do other things and needed). I would be the guy that figured out how to execute specialized UI and how to do it as inexpensively as possible (resource wise). Engineering would do the actual wiring/functionality. Tech Art was the functional role I had there. Currently, my team is much smaller. I still do the same sort of thing though, just a bit wider. We have a killer UI/UX guy, who does design side of things. I handle some design, some art, all the vfx/animation and most of the structure and some of the engineering. But is a small team, about 8 folks total, where previously our team was 50.
Ultimately, you kind of have to structure things around your team’s skill sets and skill mixes. Any UI/UX designer worth a damn will know how to technically set things up, manage assets, and work within Unity’s UI. I would say that is an expectation. They should also be able to effectively communicate what they need done to the engineer(s), if they aren’t writing code. If an engineer is not visually oriented, they should be able to follow detailed instructions/examples.
Mainly the trick is find out where the skill break is, and over-communicate at that hand-off. Also, you can have the engineering side build things in way that passes back control to UI. For example, if you want some sort of animated transition, have the engineer set up everything, with placeholder animation. Then UI can just go in and adjust things. We have occasions where engineering is ahead of us on certain features, so we will provide a grey box layout, they can do all the wiring, and we then just go in and polish that. Or if we are ahead, we’ll build the whole thing out, and they will just do the wiring.
Again, things don’t have to be set in stone, it will depend on skill sets, and may vary from feature to feature depending on how much work each person has on their plate. A UI person may not be able to code (most do to some degree), but they should know their way around Unity expertly, and be able to build animators, particle systems, etc. And a coder may have no visual taste at all, but they should be to follow well written instructions/examples. On larger teams, you can have folks like Tech Artists who specifically bridge those gaps.