I get the same feeling when switching between Gitlab and Github. The latter is a joke of usability and discoverability.
Tiny details like thread filtering, thread replies not appearing as separate comments, "show changed lines" when a new commit modifies the subject of a comment, etc. Those add up to make gitlab much more usable.
It's funny, because GitLab feels pretty bad. It's slow, there are weird places where things require a full page reload, and it's extremely aggressive about hiding files that have significant changes (yes, small changes are better, but I still need to review big ones some of the time).
Tiny details like thread filtering, thread replies not appearing as separate comments, "show changed lines" when a new commit modifies the subject of a comment, etc. Those add up to make gitlab much more usable.