No, that's just a disclaimer to prevent people from randomly installing it without thinking about consequences.

If you actively manage your Immich instane, and regularly read the Changelog before updating it, then you'll be fine.

Just use it alongside a backup.

The hardest I had to do was to change the docker compose file and update my postgres instance. As long as you don't blindly update its fine. There's the occasional bug, but so far never had issues with the data