I installed C8 on a new computer (mac mini 2012) running Yosemite. And it worked. Until I installed Halion symphony orchestra, then it crashed on startup. I reinstalled cubase. It worked. Installed EZdrummer. It crashes on startup.
After a lot of experimenting I found out that everything worked if I uninstalled the small program cDock which changes the look of the ugly new dock in Yosemite.
So if you want cubase to work with extra vst instruments you probably don’t want to run cDock.