I never use passport at GE preclearance in Canadian airports. As per post #39, the preclearance facilities accept them. I have never had issues, nor has anyone told me different. GE at other point of entry will not accept the NEXUS card.
I always have passport with me, always tucked in RFID-proof case deep in the bottom of my bag. I literally have to verify it is there periodically because I am NEVER asked for it by either US or Canada. However, I would never want to be unable to produce it.
Daughter (US/Cdn) was unable to use card in machine because the card did not have a US passport attached to it. US will only link US passport to GE if one has US citizenship, I guess. Had to use Cdn passport. Once she got US Passport, the card in machine worked.