I've never actually played as Spain, whats wrong with their UA?
Compatibility? I'm not following you. Considering how integrated faith, espionage, and all the new units will be to G&K I don't think there will be a cross-game multiplayer but even if there is, if they can change all the features for a multiplayer match how hard could it be to change the UA back?
I personally think the Aztec UA is a little underpowered but from a practical standpoint it doesn't make sense. The sacrifice of captives was a religious ceremony, not a cultural one. It was prevalent throughout Mesoamerica. It didn't generate culture - the Aztecs didn't write poetry (from what we know) about the practice. Sacrifice giving faith is a no brainer - more so than this Celt thing which seems pretty arbitrary to me.
http://www.azteccalendar.com/