Up until now Gitlab has been very unstable. 5.X promises to be much better on that front because they are replacing gitolite. The Gitlab <-> gitolite synchronization is the cause of most of my grief.
Any popular project that is written in Rails, like GitLab or Redmine, typically gets tricky to install real quick because of all the third-party dependencies. This is particularly true if the person installing it is not a developer or it is installing it on Windows or OS X. Gitlab is a bit more trickier than usual because it needs ssh configuration, etc. We are always looking for feedback on how to improve the process