So while they still have improvements to make it would be a lie to say they haven’t improved at all.
Also, I don't think GitLab has had a long downtime recently. At least not for any of my projects.
That mostly depends on whether you're using CI/CD I'd think, that's had some day-long outages/problems lately. Of course, GitHub doesn't even have it's own CI/CD, and GitLab's is amazingly flexible, so it's still the better product. But it'd be nice if it were more stable.
(Note: all this is on GitLab.com. If you self-host, it's presumably much better.)
(I am using the free tier though, so this is more informative than that I'm complaining.)
However, I re-valuated and did migrate about 2 years ago and it has been fine during that time. There have been a few hiccups, but not for more than an hour or so. I've had a team of 4-7 devs working in it all day for the last two years and we have not had performance problems. We run our own CI runners as well, and while the cloud runners do often have delays, I've never had issues with delays to my own runners unless they were all busy.