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
What is good about NixOS (and GuixOS) is that they apply to package management the same principles that Git applies to managing source code. The Nix store is basically an append-only database (you might even call it a "blockchain") of inter-dependent packages.
So from an individual computer user's point of view, it is much safer to install and roll-back software with Nix than with an ordinary package manager that might allow you to accidentally delete package dependencies and break your system. With Nix, you can install packages that actually do break your system, but because of the append-only nature, you can actually roll-back the install automatically right from the Grub boot menu, no need to re-install anything.
Another advantage of NixOS, though this is more from a system operator's point of view, is that you can guarantee reproducible builds. If the package you have installed has the same hash on all of your computers, that is a simple, human-verifiable proof that all of those systems are running the exact same build of the software. You can probably see that this is very useful for people running servers, like compute clusters, or doing things like A-B testing.
(don't know if arch supports this natively now but) declarative package managment is why j started using it... having ansible/terraform basically be a part of the os is great for me because a reinstall of the current running system just means i copy my configuration.nix and i'm back to where i was but fresh...
another thing is build isolation (you can have clashing dependencies without issues because each package specifically links to the dependencies it needs)... it does kind of bloat the disk a bit, but it also shares dependencies of the same version across packages so it's not like flatpack (if i understand that correctly)