I've been curious about NixOS for quite some time. Reading about it I couldn't see how the config sharing capabilities, setup, or rollabck would be better than Arch and sharing the list of installed packages, using downgrade or chroot.

So I decided to run NixOS in a VM and I'm still confused. An advantage I can see for NixOS is its better use of cores and parallel processing for packages install.

It's clear that I'm missing something so please help me understand what it is.

Edit: Thank you to everyone in this great community! It's always so nice to have a constructive and sane discussion.
After reading so many comments, they all confirm what I've read before and I may realize that my real problem is already having a stable system and no need for the great NixOS options that are very neat but would not benefit my specific and simplistic needs. That being said I can't refrain myself from being curious and will continue testing NixOS.

The need for only 2 config files is the top of the iceberg but hiding more complex configuration to rely on. Not that I really have too much spare time but I do enjoy learning and tweaking NixOS. With its current development state, things are changing a lot so it can keep me busy for months. That's probably what I was mostly looking for: another toy to play with.

Along my journey I will learn a lot about NixOS and may find a feature that will motivate my switch to it. Thanks again for all your precious feedback!

I'll also take this opportunity to share the best help I've found so far to start with NixOS: https://github.com/MatthiasBenaets/nixos-config And his 3 hours (!) video: https://m.youtube.com/watch?v=AGVXJ-TIv3Y