I have 2 systems running Nix, and I'm afraid to touch them. I've already broken both of them enough that I had to reinstall from scratch in the past (yes yes - it's supposed to be impossible I know), and now I've forgotten most of it. In theory, Nix is idempotent and deterministic, but the problem is "deterministic in what way?" Unless you intimately understand what every dependent part is doing, you're going to get strange results and absolutely bizarre and unhelpful errors (or far more likely: nothing at all, with no feedback). Nix feels more like alchemy than science. Like trying to get random Lisp packages to play nice together.
Documentation is just plain AWFUL (as in: complete and technically accurate, but maddeningly obtuse), and tutorials only get you part of the way. The moment you step off the 80% path, you're in for a world of hurt, because the underlying components are just not built to support anything else. Sure, you can always "build your own", but this requires years of experiential knowledge and layers upon layers of frustration that I just don't want to deal with anymore (which is also why I left Gentoo all those years ago). And woe unto you if you want to use a more modern version than the distribution supports!
The strength of Docker is the chaos itself. You can easily build pretty much anything, without needing much more than a cursory understanding of the shell and your distro's package manager. Or you can mix and match whatever the hell you want! When things break, it's MUCH easier to diagnose and fix the problems because all of the tooling has been around for decades, which makes it mature enough to handle edge cases (and breakage is almost ALWAYS about edge cases).
Nix is more like Emacs: It can do absolutely anything if you have the patience for it and the deep, arcane knowledge to keep it from exploding in a brilliant flash of octarine. You either go full-in and drink the kool aid, or you keep it at arm's length - smiling and nodding as you back slowly towards the door whenever an enthusiast speaks.