I travel to YYZ and back several times a year, going back to when I moved to the US over 10 years ago.

To be quite honest, I would never, ever, travel with a Nexus card only. Some reasons:

- Airlines may not accept the Nexus card. I have not tried scanning it into the check in kiosks in YYZ or elsewhere for years, but know for certain that when I did try it multiple times before I stopped trying, they were never readable (by any airline kiosk), even though the machine-readable code appears to be the same as on a passport.
- While immigration/customs authorities may consider the card valid for entry, airline officials may or may not. IME, even before the UA YYZ staff got outsourced, they insisted on a passport and wouldn't take a Nexus (and I traveled through at least once a month, and knew many agents by name). I doubt they'll accept it today.
- Even using it as your only document to pass through the border is a bit risky. The language I've seen over the years have never been conclusive, and most I've seen refer to a Nexus being valid in a kiosk only. That means if the kiosks are down for any reason, or you get sent to secondary, you are no longer using a kiosk, and I'd say I get asked for Nexus card and passport almost everytime I've been sent to secondary over the years (either because I've declared something, or random).
- It's just easier to have a passport with you - if you don't get asked for it, its no big deal, if you do and you don't have it, even if you are right, in an argument between a traveler (even with a page from a website) and a immigration/customs officer, the officer always wins.

Of course, you can take the advice in this post or thread overall, and do what you want with it - it is up to you. Personally, its not worth the risk, especially since it takes virtually no effort or space to have it with you.