Long-time (by now) Nix user here. It's even worse than a few hours of debugging. In my experience it requires continuous maintenance, options tend to be deprecated and moved around on a regular basis (at least in NixOS and home-manager), things like NeoVim break regularly as a result of friction between the immutable world and a lot of plugins expecting a mutable world, etc. I do install my Mac from scratch every 6 months or so (I like a clean system), but with a dotfiles repo and Homebrew, it takes me about an hour to set everything up again, which is far less time than maintaining a Nix-managed system.

For me Nix/NixOS is by far the most effective for deploying servers, development VMs, etc. I do this regularly, so it pays off - I have a familiar environment completely set up in minutes. Another place it pays off, even on macOS, is for development environments, especially if you have to pull in a lot of native dependencies (which can happen in mixed Python + C++ projects, mixed Rust + C/C++ projects, etc.).

Nix shines in all difficult cases, like setting up complex cross-compilation environments, building for old glibc versions, etc. You set it up once and then it's really easy for yourself and other project contributors to get the same build environment.

Like any tool, a good engineer knows when to apply it.