Figuring out whether the upgrade is safe is a hard problem. I've found renovate most successful in frontend JS projects where you have a ton of dependencies all with new versions coming out all the time, most of which are non-breaking, and where the danger of a bad upgrade is not that large.

For backend work or once you've gotten the easy stuff out of the way you really need to review the changelog, assess the risk, and do the upgrade safely (disclosure: my startup Infield is in this space).