GitLab team member here. We have Code Owners [1] and recently introduced Merge Request Reviewers [2]. Our team is working on the author-review handoff [3] as part of their work to improve the overall merge request reviewer experience [4]. I'm sure they would appreciate any detailed feedback you could share on the epics linked below.
1 - https://docs.gitlab.com/ee/user/project/code_owners.html#int...
2 - https://about.gitlab.com/releases/2020/12/22/gitlab-13-7-rel...
Code Owners don't work. If you assign a team as the owner it doesn't do anything unless the team itself is also added to the project. Further it doesn't seem to properly notify people either. We've fallen back to emailing everyone manually whenever a pull request is submitted.
I have the exact opposite experience, and it just works in ~90% of the cases, ~5% being behind a feature gate, 4% behind a paid tier, and 4% bugs.
It's easy to manage either via Helm or omnibus packages, and does a ton of things well enough. It still has significantly more features than GitHub.
Further when something gets reported to GitLab they automatically close old issues and things never get fixed. One outstanding huge issue is that if you mark certain files as owned by a specific team for example, it won't notify the teams to come in and actually review the code. There's options that say they do it, but it never works. (The notification system in general is an utter mess.)
GitLab is an example where everything is chopped up into tiny pieces and nothing works across different parts of the codebase. I don't envy anyone that works for GitLab as just the look of the code shows what a mess the backend must be.