Unless there’s a very specific business case that requires a custom UI it’s not worth the hassle. I want to be delivering value for the business and for users, not maintaining a UI library.
One place I worked at had built an entire responsive CSS framework, which was hard to use and took a lot of maintenance. I threw it all out for Bootstrap (as was the style at the time). Some of the senior devs were upset I’d killed their baby, but everyone else was able to move so much faster.
I'd love to have a designer that started with a style guide and then actually stuck with it. Writing CSS isn't hard, and sticking with a known set of rules makes it even easier. But then this one component needs a slightly different font size that doesn't match up to any of the established typography rules, and this other spot needs unique padding, and and and I end up having to waste so much time looking for these little surprises.
First things I stress to devs I’m working with are, here are the rules for breakpoints, type sizes, colours, spacing etc. If the designs don’t match the rules, go with the rules, not the designs. If things don’t look right let’s talk about it.