A release candidate isn't a release, and the release may have breaking changes from the RC, and obviously an RC of anything that would be a semver major release may have breaking changes from the previous actual release. So, in either direction, an RC may have breaking changes.
Ideally, the last RC should not have breaking changes between it and the actual release, but we wouldn't need a "candidate" if we knew things would be ideal.
It's not acceptable after an alpha release, It's especially not acceptable after a beta release. And it is offensive against developers to have breaking changes from the first release candidate and onwards.
Even for major releases, breaking changes is a VERY BAD thing. But sometimes it is unfortunately unavoidable.
Projects with lot of breaking changes for minor releases should raise red flags, so steer away! Instead of wasting valuable time being bleeding edge, take a deep breath, turn your focus 180 degrees and say to yourself: I think will check it out in 5 years. There are a lot of other great, stable and mature web frameworks out there.