Immutability.
I'd imagine this solution inherits any problems that pacman has. The Arch wiki states that "if two packages depend on the same library, upgrading only one package might also upgrade the library (as a dependency), which might then break the other package which depends on an older version of the library." (https://wiki.archlinux.org/index.php/System_maintenance) This is one of the problems Nix does not have by design; in fact Nix lets you mix and match packages painlessly.
This also doesn't seem to allow unprivileged users to install packages, which is kind of a side benefit of using Nix.
Docker-esque solutions' issues with reproducibility are well known; I look at https://godarch.com/concepts/images/, particularly how packages are installed:
#!/usr/bin/env bash
pacman -S steam --noconfirm
Also, frankly, I don't want to run my personal computer like a server, with complete immutability and the need to build fat image files every time I want to try out an additional program. That seems to me like a workflow that's better suited for servers where spending a few minutes for a deploy isn't a blocking operation, and where stateless service design is considered best practice.
The samples seem to be targeted at endusers/personal computers (with images like 'gaming' and Steam mentioned), while most immutable systems I see are deployment/server environments.
I've dabbled in Nix before and Darch _could_ be interesting. Can I read more somewhere?
https://github.com/pauldotknopf/darch-recipes
I think the documentation does a good job at giving you an idea of how it works. It doesn't take long to get through.
The "why" is exactly the same reason NixOS exists. I want a machine that can be declared and rebuilt deterministically (at least semi-deterministically, rolling distro and all). I looked into NixOS, but the DSL was too much, and the npm/pip/etc stuff was a mess. I am a fan of Arch because of it's package availability and documentation, so I figured out a way to combine the two, using a "Docker-ish" approach.
My entire machines are built on Travis-CI and pushed to Docker Hub. Once I make a change to my recipes, ~20 minutes later, I can pull a fresh image and boot again.
Another thing I didn't like with non-declarative OSs (non-NixOS) was that if I wanted to just test a package out, after removing it, it would leave shards of config/package dependencies still on my system. With Darch, each boot has a tmpfs overlay, which means I can uninstall/install to my heart's desire, knowing that only things I commit to my recipes will be persisted. For example, I was trying to setup Ruby, and I had to try many ruby environment managers before I found one I liked. After a reboot, I was certain that the other ruby packages I tried where 100% scrubbed from my machine.
I also like the Docker approach, because using layers, I can quickly switch our the "desktop environment" layer to i3/plasma/gnome/etc, or my base image from Ubuntu/Arch/VoidLinux. This makes distro and DE hoping a breeze.
As for using Darch as a server, I would wait until I get the Ubuntu image done. That way, the builds will be more deterministic (instead of using rolling distros). I can see using that for servers, or IOT devices. I also intend to add PXE support to boot these images from the network, making it easy to manage the operating system on a fleet of devices. In summary, it is really up to your recipes and what operating system you choose.