[microtonal]

With immutable distros, the upgrade becomes more of an atomic operation than what is offered by more incremental package manager updates.

Nix offers the same guarantees. The point was that you don't need the whole system als a single unit (e.g. an image). A system can also be a tree of immutable output paths in a store (where a single output path often, but not necessarily, corresponds to a package).

In that model a system is basically an output path in the store (in reality it's a bit more complex) and has other output paths as transitive dependencies.

Upgrades/downgrades are atomic, because they just consists of selecting a different output path that represents a system. Upgrading creates a new output path that represents a system (either by downloading from a binary cache or building that output path) and booting into that output path. Rolling back consists of booting into the previous output path that represents a system.