This is not true.
I have never encountered a stable computer clock (osx or windows) in 25 years of working with computers and music.
The only reasonably stable master clocks in live situations (live sets up to 7h long) are older MPCs or Octatrack. This comes from years of first-hand experience and is also easily explained: the system architecture is streamlined for this job, unlike personal computers, which will always have intrusive processes running somewhere, no matter how much you tweak your settings.
The more laptops in a live/stage setup, the more resyncing you will have to do during the course of the night.
External clocks won’t change that, the weak link is the OS.