SL is reasonably stable IMHO, but not rock-solid. While difficult to crash, VirtualBox and any other having modules so close to the kernel don't help with stability.
How Mavericks can be more unstable?
I don't use VirtualBox and until Mavericks I didn't even know what a kernel panic was.
I've experienced more app crashes in Mavericks, issues waking from sleep, and problems with wi-fi connectivity. That doesn't mean it's an inherently problematic OS, I've personally just run into more problems with it than older versions of OS X.