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.