IncusOS is another - read-only root FS, interactions with the system exclusively through the Incus API, no package manager, blue-green OS updates (à la Steam Deck / Home Assistant OS).
Talos on IncusOS is likely a very interesting stack that I intend to play with hopefully in the near future.
How would you say it compares to nixOS?
They are vastly different. Incus is aimed at providing a minimal, immutable Os for the hosting of VMs & containers. nixOS provides a full linux OS that is reproducible and declarative.