Yes, you are inheriting the nuances of whatever package manager you choose. Maybe another distro can give you a truly fixed package versions? You can also use apt to pin versions. This isn't something that Darch introduced, but also, it isn't something it solves. So yes, if you need 100% deterministic, Nix is your guy. I don't think this is a big issue though on Ubuntu systems, or most non-rolling distros. Their apt updates are typically well tested and don't bump major versions.
> need to build fat image files every time I want to try out an additional program
You can install your packages and use them when you like, without requiring a rebuild. Hell, that is even part of my workflow for some applications like docker. I have dotfiles with a "install-docker" and "install-vmware" aliases that installs it whenever I need it, instead of baking it into images.
> 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.
I disagree. If this was the case, then why Nix then? Obviously, stateless is valuable. I have multiple machines that I perform common tasks on. I hate having to always manage the updates when I get back to each one of them. With Darch, I can deploy one single image to all devices, and be confident they will never drift in installed packages/configuration. I never again have to ask my self "what machine am I on again?". Stateless may be the definitive way to run servers, but that by no means restricts it to only servers. I have been running Darch for a few months now, and I find it incredibly useful and calming.