I won't claim that my experience is representative, but what I've seen is closer to 50% being false positives, and then maybe 20% of the remainder being bugs but not vulnerabilities (aka lower priority bugfixes). The suggested patches have a much worse hit rate though, to the extent that I accepted only a small fraction after reviewing.

That said, an automated system generating only 30% true positives is probably still ahead on ROI as long as we're talking about tens of findings and not hundreds. The amount of human time spent investigating things that turn out to be false positives being the main difference for how over or underwhelming it is.