I recently got my Nexus approved (had my interview approx 2 weeks ago) as an Indian citizen with Canadian PR and US B1/B2 visa. This is what I was told at the interview-

(1) Land Border: If I don't have a valid I-94 then I can still use the nexus lane but I have to inform the officer that I need to go in and get my I-94 (pay 6 USD and valid for 6 months). Not doing this can get me in trouble as I would be entering without a valid I-94. On the other hand, if I already have a valid I-94, then there is no need to link it with nexus if I am using only land border nexus lane. I have to always carry my passport and PR card along with my nexus card and have the passport with I-94 page open so that the officer in the lane can check it if required.

(2) Air travel: If I don't have a valid I-94 linked with Nexus card then I can't use global entry kiosk as I would have to get my passport stamped and that would require going to see a officer at the booth. But if I have a valid I-94 and I went to the Nexus office to get it linked with my nexus card then I can use the global entry lane and I have to use the B1/B2 visa to login.

My experience so far:
US side: I used my nexus card to enter US by land on Friday and I already had a valid I-94 form attached to my passport but not linked with my nexus card. I had my passport open showing the I-94 page and the officer took a quick glance at it and asked me if I had anything to declare and reason of visit and waived me through. The entire process took few seconds and had no issues at all.

Canadian side: Returning to Canada, used the nexus lane and officer asked me if i had anything to declare and waived me through without asking to see any documents.