GitLab is great but I believe it depends on the need of the project, here are a few examples of the things that might an issue

On GitLab CE, you don't have the possibility to see the history of edited comments.

Indeed, if you need to manage a community or even to simply have full traceability, this is an important feature that is missing.

Furthermore, the ability to approve an MR is something that I miss using GitLab.