As a Home-assistant core user, is it a downgrade to use home-assistant OS?

I’ve an Odroid N2+ that I’m thinking to put home-assistant-OS on. Currently I’m using Debian on my odroid and thus I can run docker etc on.

I read that ha-osalso uses Debian and I’d assume installing docker, pihole there is no problem?
Do I miss out on any functionality going from Debian/core to HA-os?

Is updates to the OS similar to regular Linux distributions? Or will it always start from a blank slate (like immutable OS)

You read incorrectly. It is a Build-root system, very minimal, locked down, and you have very limited access to it. It is an “appliance”.

That would be a mostly incorrect assumption.

The only access you have to extra containers when using HAOS is through “add-ons”. PiHole and AdGuard are available as are a lot of other core and third party containerised add-ons.

OS updates are released form time to time and are managed through the HA graphical user interface.