not sending that much mail, but noticed after setting up SPF/DKIM/reverse DNS that the mail I sent to myself didn't end up in spam folders at gmail and hotmail etc., without having to send via a service like authsmtp.com.
If you can live with delayed messages there is not really a need for fail-over as most server will retry multiple times (default iirc multiple days). But you should have at least another email address available for example to communicate with your provider.
6. Some (most?) hosting providers are black listed and getting other email systems to recognize you as a valid agent is getting tougher and tougher.
7. You have to provide your own fail-over, or purchase it from someone else.