Debian unstable running here since about 7 years. I tend to reinstall when getting a new computer. Once I had X break and I had to set "GDK_BACKEND=x11" and "XDG_SESSION_TYPE=x11" to fix it. Something related to Wayland I guess. But I use startx directly, no login manager, so probably not a common issue.

Sometime more than five years ago, there was some trouble always when a new version of GNOME trickled to unstable one package at a time and having part of packages the old version and part new caused a lot of small issues, like thumbnails not working in Nautilus etc.

I don't remember any other relevant issues. apt-listbugs helps avoid a lot of issues. I use aptitude as package manager (commands, not the menu UI) and often use "aptitude forbid-version" to skip a bad version, not sure if apt-get or apt these days provide something similar. apt-listchanges is nice too, although I rarely see something relevant to me specifically there.