Nothing structural about the way NixOS works at a technical level prevents it from being as stable and battle tested as Debian Stable, but it isn't in practice by my experience.
I would bet money that most people who are singing praise for NixOS have sunk significant time into absorbing the whole NixOS way of doing things, learning the Nix language, tinkering with their machine config, diagnosing non-trivial problems, etc.
It is just a smaller community and younger project. NixOS is nowhere neat as stable. The security story is, comparatively, simply not present (e.g. there is no equivalent of https://www.debian.org/security/ that I can find). The user base is tiny in comparison. Packaging up stuff for NixOS often requires non-trivial patches, downright hacks, or wrapper scripts for binaries that can break assumptions made by upstream and lead to little paper cut issues.
NixOS is many things, but it isn't a Debian Stable. They fill different roles. Debian is a mainstream distribution, and the problems you'll have will be similar to the problems most people have, so you'll be swimming with the tide and be able to find help. I see NixOS more as a rolling release distro that gives you a mechanism and programming language to control configuration and rollback, but a lot of those 80K NixOS packages are just kinda sitting there packaged up in a way that kinda worked at some point but isn't really solid.
I use Nix as a package manager on top of Debian, to bring in development tools, etc. This is relatively easy. When I tried using it as an OS I had to spend way too much time figuring out weird issues.