Eventually you have to trust something. I think it's reasonable to keep backups and trust that VersionPress itself won't break. Furthermore, given the backup includes the database and the code, seems like you could test VersionPress upgrades fairly easily from the backup in a separate environment.
The biggest potential hangup I could see is something like a hacked-up theme or plugin that breaks in a way that makes the WP backend inaccessible (the infamous "white screen of death": http://codex.wordpress.org/Common_WordPress_Errors). In this case VersionPress wouldn't be able to save your bacon, because you wouldn't be able to get to its UI to back the change out.
But on the other hand, you'd have to do the exact same thing if you didn't have something like VersionPress installed, and VersionPress might save you from a host of lesser problems that would otherwise be annoying to deal with.
We'll certainly have a way to restore the site even if it is so broken that you can't access the backend. Think something like external command line utility or, for hosting scenarios, a PHP script.
