They seem to be consistently taking right decisions at the micro level. Their CI/CD design and execution is way more usable and reliable than <name suppressed> pipelines (still no manual stages, still no re-triggering, etc). Their design and integration with Kubernetes is also a great choice.
So on the micro picture, things are quite good. On the macro picture, they are building a universe. On the Issues front, they're trying to be like Trello (and in some places, reminds of Jira). They're trying to tie Issues with customer support, getting slightly in the way of Zendesk/Freshdesk. They're building deployment. Then monitoring - they support Prometheus. Now post-deployment/post-monitoring. And of course, they are competing on core git hosting as well.
Of course, they'll do a great job. Heck, if they get into the messenger domain, they'll pick a nice strategy and just integrate deeply with Slack maybe.
But the core of the problem is: None of these are instrumentable/hookable. Want to enforce some of your own organization policies before deployment? Want to use everything _except_ with your own monitoring tool? Sorry. If you use GitLab, you use their universe and everything that comes with it. There is no graceful integration with a broader set of tools.
Maybe for a small startup, just "doing things the GitLab way" and subscribing to all their micro choices (which are good) make sense. But organizations grow, and they'll outgrow these micro choices sooner than later. Then the lack of extensibility, hooks, etc. will bubble up.
All this is assuming GitLab continues to build out a perfect application platform with all the right choices (and all the version and time matrices of those choices). Hugely laudable work so far though!