It’s an immutable distro akin to Fedora Silverblue, which means it’s theoretically extremely solid but you can’t easily change the base system.

And while I haven’t tried it myself, the thing that seems to set VanillaOS apart from something like Silverblue is that is built around the idea of containing various subsystems based on other distros (like an Ubuntu subsystem and an Arch subsystem), which should make it easy to install packages from a variety of distro (or even the AUR, for another example) on top of a very solid, static base system. Under the hood it uses a container management tool called Distrobox to achieve that, but it seems to be pretty nicely abstracted for user simplicity.

I daily drive Fedora Silverblue and I do something similar with distrobox for things that don’t make sense to install as Flatpaks. In other words, on my system I have an immutable base system (with optional package layering, rollbacks, rebasing, etc.), then flatpaks or appimages for most simple applications (firefox, blender, krita, etc.), and finally distrobox to handle various dev environments and music production environment (which relies on wine and a lot of plugins).

VanillaOS is something like that, but out of the box, and aiming to be GUI-user friendly.