My previous job used GitLab. I agree that it's less polished and more clunky, at least for the overlap of features it shares with GitHub. (Mostly code hosting and collaboration m.)
I think where it shines is as a DevOps platform. I'm not sure I would say any of the features are best in class but there is value in centralizing them in one place. It takes a lot of investment to get that value though.
It could just be my lack of familiarity, but it feels less polished and more clunky.
Both work similarly enough it doesn't really matter that much to me.