There are minor bugs, but AFAIK nothing severe that would affect the OS stability. Only time I've managed to make the system crash was while overclocking the phone.

N9 doesn't need rebooting unless you're doing some major hacks or upgrading the OS/kernel. I see no reason why one couldn't get year+ continuos uptime if one is careful enough not to let battery run dry and knows how kill/restart processes if necessary. (i've encountered one networking related bug that required this)