Yep tinkering with the system is probably the main issue (for that NixOS is awesome btw.). But even when you’re not constantly tinkering. System-State accumulates over time, bugs are also apparent in (upgrading of) distros, and the maintainers of a distro cannot realistically handle every upgrade time-point x -> y, so stuff will likely break after some time.

But even when I have fixed all the issues in my previous at some time broken distros, at some point it just feels good to have a freshly installed system without all that dirty accumulated state (NixOS + impermanence and you’ll have that every reboot :P, see also this)