The way I see it, if a console dev (that have fixed hardware) making an AAA game doesn't use all the available hardware, then they are seeking parity even at the cost of not fully utilizing the hardware.
Basically if you make a game exclusive for PS4 (especially for AAA game), surely you try to use all the available hardware. If in the event that you're limited to the CPU, doesn't mean that you stop exploiting the GPU (or maybe the extra bandwidth or something else). If I were to make a game that really CPU limited that I can't fully exploit other parts of the system, I certainly would tweak my CPU usage so the approach can be more balanced towards other parts of the system.
In Unity case, if the game is really CPU limited, it means that on X1 it might not utilize the full 12CU? Or it just happen that in this CPU limited scenario, they can only use 12CU? Isn't that convenient for X1? If the dev only make Unity for PS4, would they still make a game limited to X1 spec or somehow magically they can use all the available hardware?
Anyway, that just me thinking. It can also be that X1 code is more optimized vs PS4 code because of MS help. But as it is right now, the game isn't out yet. It should be fun to see how much parity they can achieve with Unity and whether their claim of CPU limiting performance can be translated to better gaming experience.