I am using C1 [not-the-very-latest] 9.1.2 on El Capitan 10.11.6 (a recent point upgrade for me) on a 2009 Imac 3.06, and find it no buggier than previous versions—it hangs on startup occasionally (every tenth time?—just guessing; an app restart always fixes the problem), and it seems to quit a bit less frequently than it used to, while in the background. The Phase One bug alert dialogue is still glacial compared to Apple’s, and it thinks it’s crashed out every time after force-quitting.
I really should download the very latest rev; I need to look at the release notes to see what’s new and ‘improved’!