Jetbrains UpSource was created because they weren’t satisfied with code review in GitLab merge requests.
What they mention as issues are good points, but are they valid?
The article is a year old, perhaps there has come some improvements on GitLab that addresses the issues Jetbrains had or still have.
We have been using UpSource for a little while now, and it is good, but has drawbacks.
You cannot do merge requests in UpSource, only code review.
GitLab has the ability to see diff between revisions made after a rebase. This is annoying in UpSource, and requires us to review all the code again when new changes are made to existing commits.
GitLab thumbs up, or reactions are not good to indicate acceptance. I have read you can assign approvers in GitLab that needs to approve it before it can be merged.
I have been trying out GitLab and really like the merge requests and I really like the GitLab Web IDE.
GitLab seems to have better integration to JIRA and CI.