It's rock solid. We ran a pretty large installation of these to control a sizeable fraction of the worlds flow of container shipping. Over the course of the project there wasn't a single outage attributable to the OS or the application itself.
I remember an ad where they mentioned a computer running QNX that was booted once and turned off once, when it was decommissioned 10+ years or so later.
Totally believable, in fact, that would be the expectation. Other than hardware failure it should just work.