I've been using Immich for a year now and haven't lost a single image.

Having said that, if you self-host something like this, you're responsible for making backups. The actual photos uploaded to Immich are stored in the file system as regular files. Any other data that Immich can't easily recreate is stored in Postgres, which it automatically dumps once a day. So if you back up your image files and your Postgres dumps, you can always recreate your library.