As a Canadian permanent resident (not US or Canadian citizen), is a NEXUS card enough to cross the border by land or air or for boarding a domestic or transborder flight ?

Domestic flight in either country - yes
Anything else you mentioned - no

From CBSA:
From CBP:

Do CBP officers at Canadian Pre-Clearance only ask you for your passport if you used it at the GE kiosk (as opposed to Nexus)? If that's the case, I think it would be a matter of consistency.

Also, can you use just your Nexus to obtain a boarding pass at a kiosk on a US-Canada flight (without having to have your passport scanned)?

Ask the airline. If the airline says you need a passport, then you need it. What CBSA/CBP say you need and what the airlines say you need are two different things.

I'm not convinced it is supposed to be that way. My strong suspicion is US privacy regulations prohibit sharing this info. with Canada unless specifically approved to do so by the member. I'd also guess submitting the application online you agree to this sharing, but not in general for the site, like when updating your passport. If this is the case, would require adding a disclaimer that you'd have to check before submitting update, or doing it as now, with a second, manual update for the Canadains.

Not my experience - UA has never taken them, IME, definitely at US stations, and at YYZ - both pre- and post- UA outsourcing its ground staff there.

CBP officers will ask for your passport if they want - shouldn't matter what ID you use at the kiosk. Typically, if you don't get an X/O, you will be waived through without being asked for anything. Typically, with an O (food declaration, get sent to booth but not necessarily secondary), I typically just get asked what I have, either from random or for declaring something), I've had officers ask just for my Nexus card or for my passport. My rule is to bring my passport no matter what, because if you're asked for your passport, you've already lost, no matter what the stated rule is.

As to your question about being able to scan your Nexus card, not in US stations for sure. At YYZ, the scanners at the check in kiosks are complete crap, and I can't even get it to scan my passport which works in every other kind of machine I've tried.

Same. Never had my GE or NEXUS card accepted by UA when flying from EWR, YVR, YUL, YYZ.

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I always travel light and don't buy anything at my international destinations, so I've never gotten an O. With a "normal" receipt (no X or O) at US customs non-preclearance, I just hand the officer my receipt and have never been stopped (even when the GE photo caught me off guard and took a photo of the top of my head [looking down at the fingerprint scanner...)

When at preclearance in Canada they typically want my boarding pass and my passport, sometimes NEXUS card as well.

YVR: Both times I've been there they just asked how many bags I checked.
YUL: Typically more detailed questions on the nature of my visit, turning the monitor towards me to ask if the bag depicted on screen is mine.
YYZ: They've just asked to scan my boarding pass and then made sure that they had a photo of my bag. [One time UA didn't photograph my bag and I got sent back to international baggage handling #2 while United looked for my bag for 35 minutes... joy.] If you don't check luggage, my observation has been that an airport employee directs you past the booth and you just hand your GE receipt to the CBP Officer near the doors to the transborder gates.

Never tried it, UA has my passport saved from mobile app scan and I travel internationally enough where it doesn't "fall off" my profile. I just enter my MileagePlus account # when I get to the kiosk.

My NEXUS card was accepted at the gate by a UA agent in DEN in September.

For a flight to Canada? Or within the US?

Great To Hear (-:
 

Got an email last week from GOES saying that this feature was coming, but not till April & only at certain airports (YVR not being mentioned as the 1st one BTW), so glad to hear this is now "live".

Yeah, I was surprised and impressed to see a government program implemented ahead of schedule!

To Canada. Sorry, thought that was clear from the context. I haven't had to show ID at the gate for a domestic flight in years.

In the past several months, CBP have been doing 3-way checks of picture in kiosk receipt, passport and you. This includes YUL where they ask for passport even if you use NEXUS card at kiosk.

Since January 20, CBP has stamped my passport every time through YVR Preclearance. First time being stamped into the US since I got NEXUS.

In today's uncertain environment, I would carry all documentations with me in case someone decides another form/documentation is required. It could be disastrous not being allowed in because of a new requirement.

I take both card and passport everywhere. This way if something happens to one I still have the other to get back. No needing to find the closest consulate and waiting.