The tricky thing with the naive copy as a database backup is that it actually could work if you test it while the database isn't writing at the moment. For example when you only tested this outside production on a test server without load.

But yes, you do have to test and verify that your backup works. It might be configured entirely wrong, the cron job might not be running for some reason, you set it up with encryption years ago but lost the passphrase. There are plently of ways this can potentially fail.