For cross-boarder shopping, the Nexus card advantage is a of a toss-up. We consider it essential for entering the US, because lines are often 60-90 minutes at our local crossings. But we rarely use Nexus for the return trip.
FYI, we always declare everything. And yes, there are very serious penalties for not declaring everything, including a lifetime ban on ever holding a Nexus card.
When you wait in the regular line, depending on how much you spent, the officer may or may not decide to charge duty. They decide whether is worth their time and effort to collect a bit of tax on items you purchased on a day-trip.
However, when using the Nexus card, you need to fill out the complete declaration card, documenting all your purchases and their country of origin. (Filling out the form takes 5-10 minutes.) This declaration card is used to charge your credit card (linked to your Nexus account) the exact amount of duty, to the letter of the law. (If you do not link your credit card to your Nexus account, you are supposed to pull over and pay duty in person, every time you import goods.) Using the Nexus lane for return trips is fine if you were traveling for several days and are within your duty-free limit. However, if you are doing a <24h day trip, you will be charged duty on everything applicable, and your credit card (linked to your set of Nexus declaration forms) will be charged accordingly.
In my experience, when we used the Nexus lane to return to Canada, we saved 50 minutes in line, but were charged $50 in duty on $250 of purchases (mostly kids clothes). Similar shopping trips, where we returned in the regular line, we have never been charged. (The only time we paid duty in the regular line was when we imported a case of wine.)
Now, we do our shopping then go out for dinner, and cross the border in the evening when the lines are shorter. The net cost is the same, but we actually feel like we're getting something for our $50.
