I think @
Adam Smith is correct with real world advice for air travel.
I do not seem to have any problems anymore using Nexus only from US to Canada other than some smaller stations. In fact UA seems to demand a passport more often. In all cases I have my passport anyway specially since I need it for the Global Entry machines leaving Canada to US.
International to Canada is a whole new discussion as several countries track only passport and visa for entry and exit. They do not care what your home country accepts, they need a passport. Examples in my recent travel include India and Taiwan and they both track you with fingerprint and exit stamps.