It has a K3S to deploy apps, defined in a certain format (both in their official repository and in custom repositories), and uses that snapshotter to manage layers in the underlying ZFS storage.
Edit: I've used it for a while, although not heavily, and haven't had problems so far.
In general as a NAS: No every function is already migrated to Scale, there are some quirks here and there, but as a NAS is working OK
Regarding the "Apps": It's too heavy for (really) old systems, but it seems to work OK in newer systems. Although you could in practice, don't expect to "just run a Docker image", you better use their "App charts". The "App" definitions are based on Helm and there is both an "Official" repository and a "Community" repository (Truecharts) of them. You can also create your own repo and add it to your Scale deployment.
I have some simple apps (Transmission, Heimdall and some other stuff) running in a E3-1265L 32GB RAM and everything es working well.
But I have a really old HP Microserver N40L as an offsite backup for my NAS where I've installed TrueNAS Scale too, and although the NAS works OK, if I try to install any apps, the K3S installation plus all the tools overhead is too much for the machine, and keeps it at 100% CPU almost permanently. If you remove all the apps and detach the storage from the App system it uninstalls all that stuff and goes back to work well again.
It got out of beta with the SCALE 22.02.0 release.
I've been using it a little bit, but I feel like they still have some quirks to iron out, especially since Scale abstracts K3s and doesn't allow users to control it directly.
* https://github.com/containerd/zfs