Disclaimer: I only tried NixOS for less than a month when I was a complete Linux noob, I have since then been daily driving Arch Linux for about 2 years now.
For me, at least on the surface level, NixOS just felt like Arch Linux, with more similarities than differences.
What was nice about NixOS was the single config file for everything, although iirc I had to reboot every time for it to be applied while with Arch you can just install something and run it immediately.
Edit: I either remembered it wrong or I was doing it wrong because you don’t have to reboot the whole system according to the reply from hallettj.
What I didn’t like however was all the packages that got installed (through the list in the config file) had really strange directories which I couldn’t find easily.
like on Arch the packages and the executables are basically all at /usr/lib/ and /usr/bin/ and iirc it was pretty much the same on NixOS, except on Arch I’ll have usr/lib/firefox but on nix it would be usr/lib/u123uadqasd782341kasjhiu3sh932s9sdasdsapzxcqw-firefox
Another thing is that it works great for everything you install through the Nix config file, but it’s not necessarily going to clean up any files created by programs that got installed through it when you remove the packages from the config file.
Like say you have installed steam and then you install some game through steam, well that game wasn’t added through the config file so there’s no guarantee that if you decide to remove steam that you will also remove whatever the programs steam installed or if they created some new files somewhere.
Of course the same thing already happens on other OSes as well, so you could say that it’s an upside that Nix is better at cleaning up after itself whenever you remove something, but also because it’s supposed to all be controlled through a single config it just feels that much worse when you have to hunt down some file somewhere.
Again these are mostly my anecdotes from 2 years ago when I was a complete noob. Maybe I wouldn’t have any issues if I tried it today. And chances are I was just trying to do something you shouldn’t even be doing.
Plus at the start I used KDE Plasma 5 on Nix and Arch, maybe it will go better if I use i3wm on NixOS like I’ve been doing for a year and half or so on Arch now.
At least I’m pretty sure that having daily driven Arch for 2 years now I would have much better chances with NixOS now than when I tried it with 0 experience on Linux.
So since you’ve already got the experience from using EndeavorOS you might not have any big problems using NixOS, or at least learn how it works pretty fast.