Well, what I am saying might as well be FUD as far as Darwin/MacOS X is concerned, because the foundation OS (Mach 3) was engineered with realtime support; the point is that I do not know whether this is currently used by the current implementation of Darwin/Mac OS X.
However, traditional unices are performing rather poorly in realtime aspects (Darwin is not a traditional unix in this sense), so I would not be shocked if there were some temporary problems caused by the BSD layer.
This should not be quite noticeable in Quicktime, apart from occasional frame drop: sound sync, if buffered, is easy; it's only 150 KBps, or thereabouts. 24 channel realtime harddisk recording with realtime effects is not as easy because sound discontinuities are much more noticeable than dropped video frames.
So, Cubase would have problems in a traditional unix, or at least most of them (FreeBSD stable would be quite useless, and Linux should not fare any better---Solaris might, though); whether it will have problems in Mac OS X remains to be seen (but it really should not, especially once DVD playback is there to prove that realtime aspects of Mac OS X are acceptable).