Gitlab's CI/CD is amazing. Of all the systems I've seen, it's the easiest to (a) deploy your own runners easily, and (b) mix private and public runners (i.e. the equivalent of using a free plan on Travis, with the benefit that for heavy-duty jobs you can run your own machines). That and first-class support for docker (no more being at the whims of what Travis chooses to deploy in their images), make it a really great system.

But we had been on Github, and that turned into something of a pain.

First we tried mirroring Github to Gitlab. This worked, but (a) it added a minute or two of latency to starting jobs in the CI, (b) since it went through one account, all the build emails went to a single person, and (c) the mirroring happened on our own infrastructure, which turned out not to be so reliable. So we got tired of this approach.

Eventually we decided to move development to Gitlab, but we didn't want to move our users, so we left all the user-facing elements on Github (issues, PRs, etc.). This also kind of works but has its own set of issues.

Things might have been different if we'd started on Gitlab from the very beginning, but as things are, I think having the repository on Github and the CI on Gitlab would be the best of both worlds for us.