The real reason you shouldn't be using GitLab is performance. How is it possible that that page took over a minute to load? Nevermind that the design is completely incomprehensible.
(But yes, gitlab's web interfaces are generally frustrating. Slow, bugged, or just organized in ways I find challenging.)
If you don't find them acceptable, that's legit. Those are the "real reasons" you shouldn't use it. However, those reasons may not apply to others. They don't apply to me, for instance, and as Gitlab has a lot of users, I'm not alone.
i use gitlab because it allows me > 1 private repo. if there are better solutions then i'm all ears.