It's fine to have an unstable backup system, as long as any failures in your backups are uncorrelated with failures in your primary system. And a random datacentre burning down probably isn't correlated with anything else, unless you're foolish enough to host your primary and backup copies in the same building.

All else equal, a more stable backup is of course better, but any backup is better than no backups, so choosing the cheapest possible option is often the best strategy since that's the one that you're the most likely to keep using long-term.