This 2019 blog post from a Debian developer describes some of the problems they are facing from a developer perspective: "Winding down my Debian involvement" https://michael.stapelberg.ch/posts/2019-03-10-debian-windin...
I'm sure there are many valid criticisms of Debian, the main one for me is overpatching upstream packages.
Distros and packaging have much lower relevance as operating systems are viewed as applications, rarely are unix systems multiuser, often serving a single software package. Even desktop operating systems are getting containerized, Qubes, etc.
[1] Which is not to say that messaging is bad - a web interface with a proper message queue behind it would be fine. In fact that's how Fedora's systems work. It's that email is not a reasonable messaging system for this purpose.
It was pretty neat in 1998. Today, it's long eclipsed by pretty much everything else.
If I were Debian, I'd migrate the whole lot over to GitLab issues seeing as they have the infrastructure for it set up (salsa). The problem is there is a very vocal minority who still think it's not only good, but the best choice for Debian. I doubt the silent majority who suffer it agree. Like anything, it's an entrenched part of Debian culture that's hard to change. But it was time to retire it over a decade back.