Except the Priest is an artillery piece, and every faction has "exceptions". The JPIV is an exception for OKW, which doesn't get medium tanks for instance.
This one is quite amusing. Because it means you did not understand what I've told you.
Let's continue with my example. When you create a class (Vehicle), every child class (Jagdpanzer 4, Puma, King Tiger) has those basic methods such as Drive or Shoot. But what you do is confuse polymorphism's effects in heiring methods and using classes' instances.
Class Vehicle is the parent class of Light Vehicle, Assault Gun, Tank Destroyer, Medium Tank and Heavy Heavy classes (let's say).
Puma extends Light Vehicle class. Jagdpanzer 4 extends Tank Destroyer and King Tiger Heavy Tank class.
Now OKW faction, which consists of units (Sturmpionieers, Volks,wagens, Stukas) extends Faction class, but there is nothing in Faction that defines amount of unit types. There is no variable or constant that tells OKW "you have to have at least x of Medium Tanks". Now this is what I'm talking about.
The reason pop cap abuse is so problematic with the Priest is that your only ever paying for it for the very short amount of time it's barraging, the rest of the time you can just have it decrewed.
Well? Is it so problematic? You choose to discard one of the most basic faction features to solve that, let's be honest, small problem? This is simply a perk of that unit.