....and this is only Google phones solely because NONE of the alternatives meet the team's stringent security requirements.
Not my preference, but they seem so far ahead of other ROMs right now that I use it still.
I do believe people have built and installed it on other devices without too much trouble, but I don't think that'll ever be supported.
They recommend Google play over fdroid due to some security concerns, not because they like the service. The reasoning is available on their discussion forum.
And yeah, it won't officially be supported on other devices than the upcoming Motorola and perhaps the next pixel. Too bad.