It really depends on the kind of failure that you’re experiencing.
I want to be clear about one thing when it comes to registry backups: they are not a complete backup solution. Even if what’s preventing you from booting is a registry issue, the only real, true backup solution to the scenario that you described is a full image or machine backup. A registry backup simply cannot save you from every possible failure mode – even those that involve the registry itself.
Which may leave you wondering, “If I can’t restore a registry backup, then what good is it?”
It’s simple, really: most failures aren’t that catastrophic. Not all failures prevent you from rebooting and restoring the registry.