I have run NixOS for about eight years on server and desktop and been a nixpkgs maintainer. Yes, most of the time I would agree with you. The fact that you get warnings in the terminal for a lot of incompatibilities and changes when upgrading is a really nice touch and upgrades tend to be smooth. I do not use rollbacks much, but when you do need them they are
really handy. Having every configuration in a single file makes you more bold to play around with configurations, which felt really empowering when I first got into NixOS, as I knew it could roll things back and I no longer had to keep notes on how each box was set up to refer to in the case of a reinstall or migration.
However, I have had one machine become unbootable as it could no longer mount its encrypted disks after an upgrade, forcing me to mount a rescue image remotely, mount the disks manually, lift the data out, and do a complete reinstall (migrated the box to OpenBSD at that time). Similarly, NixOS once messed up systemd (or vice versa) so badly that I could not even reboot without forcing a power cycle. Lastly, I have had a package break for my use cases by maintainers enabling so many custom flags by default for a package that they enabled one I have never seen enabled by any other packaging team and that then broke RTSP in "funny" ways. Ubuntu did tend to break things like graphics between releases at times back when I used it, but I have never had any other distribution or operating system throw curve balls like the three things I mentioned here.
My general impression of NixOS is that the core is solid, but that nixpkgs just has such a large number of things that it supports that the maintainers struggle to test them all and can not anticipate the interactions between all the packages and options. The default Julia package being so broken that it produced incorrect mathematics due to nixpkgs' insistence on allowing you to swap out the Blas library and also having turned off the unit tests for example springs to mind. This was shipped to end users for a long time before I noticed it by accident by enabling the unit tests and stepped in to clean it up. It all feels very "Gentoo", which was indeed an inspiration for NixOS by the way.
Now, return to that last sentence in the first paragraph that I wrote about feeling empowered to tinker, ultimately, I feel like you should try to resist that urge as it is what pushes you into the untested fractal of possible configurations that NixOS allows you to explore. My other main operating system is OpenBSD, where the mentality is "Stick to the defaults or suffer the consequences"; with NixOS, I feel like everyone's box is more or less a tailored suit, which comes with both its ups and downs.