In 2014, after a stay in the Maldives, we were flying MLE-SIN-ICN-YVR, the first two legs on Cathay Pacific and the final leg on Air Canada.
The Cathay Pacific gate agent demanded proof that we two US passport holders were admissible to Canada. Argument got us nowhere. After talking about NEXUS, we pulled out our NEXUS cards and that satisfied her, even though I doubt she even knew what NEXUS was, other than what we'd told her.
Without those as our talismans, I'm not sure what would have happened.