I think my distrust of the First Alert comes from this video I watched in the past:
https://www.youtube.com/watch?v=W8YP9CWek5I
So, reading through the ZCOMBO and Nest Protect manual, the ppm alert requirements for CO are listed at the same, but I trust a hard-wired alarm more than a battery powered alarm. I don't have any evidence to say a Nest Protect or hardwired is better, but I have a hunch it is. Totally willing to admit I could be wrong, however. Consumer Reports consistently rates First Alert at the top, and Nest Protect gets dismal ratings from them.
But also, the main clincher for me is the fucking battery chirp. I'm sick of getting up at 1 to 3 am in the morning while I walk around searching for fucking smoke alarm chirp (can you tell that this really pisses me off?). I've had too many bad nights of sleep while raising babies to want to do that ever again. I do still have a First Alert ZCOMBO in my office, though.
Also, I still have one of these next to every bed. And also try to remember to take it with me when traveling to my in-laws house that stick us in a basement room without a smoke detector, where all their smoke detectors are probably 20+ years old and they use a wood fireplace to heat the basement.
I use this:
https://www.amazon.com/Kidde-Battery-Operated-Monoxide-KN-Copp-B-LPM/dp/B004Y6V5CI/
Maybe it isn't designed for planes, but I have "tested" it in vehicle exhaust flows and it does work and relatively rapidly, and it's very loud. I just keep it in the back seat openly exposed to cabin air.
I like that you can see the readout at any time just in case.