Just adding to the conversation here from our own recent experience. We returned to the US a few days ago from a 9 week cruise in Canada. Coming back to the US, I filled out the info on the CBP ROAM app, and then found this thread with the press release referenced above by Curt/Red Ravin. We both have NEXUS cards so I called the number instead. After waiting on hold for some time, an agent responded. The first thing he asked was whether I had the CBP ROAM app. I said I did but then quoted the press release that says to phone in. He said, "we prefer you to use the app, then if you have trouble, you can call this number." I submitted our info via the app and we had our clearance about 10 minutes later, no further questions asked. I suspect the process would have been a bit longer if we had not had NEXUS. One other point which is not made clear in the app: the app sends your coordinates; if you are not in US waters when you make your request, they cannot process it.
When we entered Canada on July 6, we used the ArriveCAN app (entering our NEXUS info) and clearance was just as painless.
John