It depends a lot on what you're looking to achieve with the phone, I think. If this is going to be your primary mobile device, you should choose something from their list of supported devices that has verified support for all of the conponents you need (wifi, gps, mobile data, sms, calls, etc), otherwise it's just going to be a pain to live with. I tried running a nokia N900 with postmarket as my daily for a month back in 2020, and while it was very cool, the quality of life was so so low.
If this is just going to be a fun project to tinker with, though, and you feel comfortable hacking and contributing to the project, you can dive in anywhere.