This is only an issue if you're not a citizen. Not commonly known but you don't need any kind of document at all to enter the country you're a citizen of. Just means they have to check further.
So if you show up at a Canadian POE with a NEXUS card and say the NEXUS machine isn't working then CBSA will just do some checking to establish you're a Canadian citizen and then allow you in.
But say for example you appear at a US POE with a NEXUS card and your LPR card has expired. In that situation even though your NEXUS card is still valid (on the face), in the NEXUS database your card will have been marked as invalid for entry into the US because your LPR card is invalid. And you will be denied entry.
The NEXUS card is a replacement for a passport at POEs that accept NEXUS cards, nothing else. It is not a replacement for an LPR card (although you may not be asked to present it, but you must have been issued a valid card). It doesn't alter the application procedure for a non-immigrant category.
And you're still at the mercy of the carrier, like I mentioned above.
Bear in mind even citizens of countries other than the US and Canada can get NEXUS cards and they don't even have to be LPRs, the requirement is only that they have been legally present in the US or Canada for at least the last three years. But in that situation it would be unwise to show up at a POE with only the NEXUS card, because if there's a problem with the card they can deny you entry because you're not a citizen of that country. The only real advantage in that situation is that you skip the line.




