Funny, I've had the opposite experience. Because Nix is so stable and allows risk-free easy experimentation and rollbacks, I'm always re-evaluating and upgrading my system. An added benefit is that the changes you make stick, and you've got a paper trail of the changes in git. You can create a whole set of changes behind a single flag or config file, and flip back and forth between different setups nearly instantaneously.

With Ubuntu I was afraid to change anything because I might break things and not know how to get back to a working state.

With Nix, you are always on a fresh, pristine system, because the file system is mostly read-only and an exact reflection of what's in your config. It's impossible to get crufty.

That doesn't make much sense because Nix doesn't prevent you from reinstalling, and after you reinstall, you are not forced to copy your old configuration.nix to your new /etc/nixos/.

In fact I think Nix is much better here. Instead of reinstalling you can just audit your configuration for anything that you don't want. You don't need to worry about stray packages, config files or services like on other distributions. And of course if you accidentally clean too much it is easy to revert via source control of your config file.