Given that it's 7 weeks since Adobe put out the ACR 8.7 Release Candidate, I would imagine that they must have had serious unexpected problems with it.
After all, a "Release Candidate" means that it is thought to be perfect, and there is the expectation that it probably won't require any change before full release. Maybe just a quick test on a larger range of machines in case of very minor issues.
By contrast, a beta product may be functionally incomplete, may well have bugs and requires substantial further testing.
Or is "Release Candidate" Adobe spin for "beta version"?