Originally Posted by PWMTrav

Fully agree with you. This particular scenario seems to come up a lot in this thread and I'm not sure why. I'm a US-EU dual citizen. Here's how I've done it, and it's never been a problem -

US -> EU
Airline Check-in: EU passport (online) or (check-in counter) both passports and let them figure it out (the airline just needs to know that you can enter where you're going)
Any interaction with a US official: US passport (Note: This has never happened to me outbound - I don't think the US has actual exit controls)
Arrival: EU passport - one family member is US only, so sometimes we're directed as a family to the EU lane and other times to non-EU, but same process

In the above, I have flown into my EU country of citizenship, as well as a different EU country. No issues either way.

EU -> US
Airline Check-in: US passport
Exit control or any interaction with an EU official: EU passport
Arrival: US passport + Global Entry

The last time we went, we flew Air France out of Seattle and they were using facial recognition for boarding - no problem. On the return, we pre-cleared in Dublin and flew Aer Lingus, no problem at all with GE. With the disclaimer that this isn't legal advice and just to the best of my knowledge, the only thing you must do is when interacting with an official of a government that considers you its citizen, present the passport of that country to them. The airline is not a government official, and the US doesn't have exit controls that require me to do anything specific (again, to my knowledge). And when ETIAS goes live, I don't see how you can do anything but the process above. Again, all to the best of my knowledge, but we've never had a problem with check-in or a border official on either side.