Chromium OS vs NixOS

Since you only have the browser, you cannot install applications (outside Chrome Apps which are decently protected by their limited API and the Chrome Web Store). This means no harmful virus, and also nearly no way you can screw your system. Everything will just work.

Chromium's system UI uses as little screen space as possible by combining apps and standard web pages into a minimal tab strip: While existing operating systems have web tabs and native applications in two separate strips, Chromium OS combines these, giving access to everything from one strip. The tab is the equivalent of a desktop application's title bar; the frame containing the tabs is a simple mechanism for managing sets of those applications and pages.

Because Chromium OS is designed for users who spend most of their computer time on the Web, it is intended for use in computers with little local storage and fast boot-up times.

Atomic non-destructive upgrades / rollback of a system upgrade / declarative reproducible system configuration / unprivileged installation of packages / transparent source or binary deployment.

You can start with a minimal environment and add packages and software to suit your needs as you go along.

NixOS is configured using the Nix package manager, allowing your system to be replicated and kept in sync across multiple machines. Great for keeping a laptop and desktop in sync.

Packages don't break after a NixOS upgrade as they are prone to with other distros (especially Arch).