[elagost]

Consistency. All the Google hardware forever has allowed easy factory unlocking without a fuss, easy ways to restore to standard OS images without jumping through hoops, and are widely available. Plus they allow re-locking the booloader and the phone equivalent of enrolling your own custom secure boot keys. They also provide firmware updates for a long time so you can get platform/hardware patches too. CalyxOS does provide these in their images.

The 3a/4a are cheap and have headphone jacks and good cameras. What's not to love? Until they change their policy on unlocking bootloaders and installing custom OSs they're great devices. I still have a Nexus 5 that runs PostmarketOS and Ubuntu Touch, and if it completely breaks I can always use ADB/Fastboot to flash the Android 6 images that are still on Google's website. Don't even have to log in to get them.

[m4rtink]

Devices supported by the Sony Open Device Program shoukd be also a good target:

https://developer.sony.com/develop/open-devices/

There are projects such as Sailfish OS that make use of this to run on originally Android hardware.