It is a fork of CoreOS, from when they got bought by RedHat and it was abandoned (or rather morphed into being fedora based).
It is has been fairly stable for me over the past two+ years, with one systemd-resolver snarfu. The auto update being baked in from the get go is nice.
Only thing to be aware of is that Kinvolk who are the maintainers have been bought by Microsoft, though so far it has not affected anything. Also if you don’t like systemd then it is not for you, as that is more or less all the distribution is.
They are running docker 20.10.23 on the stable branch atm. https://www.flatcar.org/releases
I am not using it with immutable config, so the only thing flatcar is doing is it has an A and a B partition for
/usrthat it switches between on updates (such that it can always rollback to the last working system).I only used ignition for the initial setup, after that I just ssh in to the machine and change systemd services via /etc/systemd/system (such as added new mounts, using systemd unit's for running docker containers etc.)
Adding a user is initially done: https://www.flatcar.org/docs/latest/setup/customization/adding-users/
The idea is that all software you need to run except for systemd and some utilities (more or less what is in busybox) are run in containers, which i think was the same deal with RancerOS.