I've started a game as Spain and noticed the Sicilian Pikemen have significantly better armour than Tercios, and only lose out very slightly on their attack stat and attack speed. In battle it's incredibly one sided, despite the Sicilians being a militia unit unlocked at a lower tier barracks.
I checked some of the north Italian factions and they seem to have a similar dynamic where their early militia pikes are much better than their late game professionals.

Is there any reason to use Tercios at all?
It feels like a mod oversight or something, my last game was as the Swiss and their pikes follow a much more natural stat progression.