GitLab is pretty impressive with the rate of features that gets shipped, but it does come at the cost of things not always been stable even though I think it's pretty impressive how stable it is considering the volume of changes being shipped.
That said I found a way to have it stable is to run GitLab on your own systems using their omnibus installer, and then upgrade around the 20th of each month before the next big release so you upgrade to a version with most of the patch versions instead of the latest bigger version.
We're actively working on making sure GitLab.com is ready for mission-critical workloads. You can see a list of ongoing efforts in [1].
However, we're not quite there yet. At the moment, for mission-critical workloads we recommend self-hosting.
[1] - https://gitlab.com/gitlab-com/infrastructure/issues?scope=al...