Good questions, particularly about the LTE. If you're in the USA and not running a stock ROM, and not using a Pixel phone, there's a good chance that VoLTE is not going to work. So you'll have 4G data, but no voice calling.

Just checked the website. VoLTE is not supported at all, and there's no aGPS, so I don't see much of a reason to use this as a phone operating system. Maybe for just a mobile data device.