Same experience with Debian. Preaching religious, unquestionable stability but a soon as you install an out of tree software, everything falls apart in your hands.
Any Debian system I used fell apart on upgrades in some unrecoverable way and it was a giant pain in the ass.
The last NixOS stable release I deployed in ~3 hours onto ~30 VMs with only minor issues and everything continued to just work.
I don't actually care about uncommon software stability on Debian. The hypervisor is Debian, so it runs very basic happy path software that always gets tested by the Debian team (and/or Proxmox team) for upgrade problems, and then all of my crazy-ass software runs in containers/VMs which are simple to destroy/rebuild if necessary.
The Proxmox team ensures that the hypervisor stuff works, and now I don't have to worry about a basic change to a nix file resulting in "Unknown entity flibblefrazzle" coming from some random place 18 levels deep in the bowels of the package system/os. It really got to the point where I was afraid to touch anything anymore.
Now I can run an up-to-date Plex! And Chrome Remote Desktop! Without spending 3 whole weekends knee deep in nix guts!
The last NixOS stable release I deployed in ~3 hours onto ~30 VMs with only minor issues and everything continued to just work.