Another concrete example is eglot: it is developed in the open on GitHub, which is great. But it is closely related to two other emacs packages: flymake and eldoc, which are developed via emacs-devel, or something, I've never really understood. So when changes need to be made in those packages it suddenly becomes very opaque, and I am not at all sure how much code review goes on. (This is not at all a criticism of João Távora who maintains all three I think! It is more structural and related to Emacs development in general.)
It dissuades me from contributing to emacs in general. There is nothing advantageous about reviewing code by mailing list when we have GitHub and GitLab. For those of us who use GitHub and GitLab in our day jobs for code review and collaboration, it is really baffling and frustrating. Furthermore, although the org-mode mailing list is, I am sure, just as pleasant as it always way, lists like emacs-devel are incredibly toxic and depressing to read; a bunch of men arguing just like they did over IRC in the 80s and 90s. And good god, don't ever go near the #emacs IRC channel if you're not someone of that gender from that generation!
I am sorry to say it but the emacs community has an atmosphere problem: it is common for people to be unpleasant in public. For example, the magit maintainer responded rudely, aggressively, and discouragingly to recent PRs against magit. Stallman's absurd one- or two- sentence replies. This is of course not Org-mode's fault; but I believe the future of Emacs development is to embrace GitHub / GitLab like other open source projects and leave the crusty 90s-era practices behind.