The true scientific issue........is actually unkown.
My theory (take THAT for what its worth):
The Ford Gold coolant requires very specific, frequent monitoring (CLICK ME
to read the Ford TSB written for this procedure), which is such a pain in the behind to stay on top of, nobody does it properly.
Ford used their Gold coolant for their own convenience. That coolant was installed in their whole product line from the Focus all the way through to the 6.0
The Ford Gold does not handle the heat that a diesel engine generates, (my speculation) and the Nitrite and other additive levels fall rapidly (hence the reason for FORD writing the TSB).
Once the Nitrite and molybdate levels fall, a few things can happen. One of which is that rust scale will be allowed to begin forming on the walls of the cooling system, since the Nitrite is a rust inhibitor.
If you have ever done a cooling system flush and saw the flakes of orange that dissolve as soon as you touch them, that is rust scale. In a 6.0, that junk collects in our oil cooler, since the oil cooler is designed much like a "P" trap. Both the oil and the coolant enter/exit through the top, so all sediment will get trapped within it, slowly clogging the coolant passages.
There is also a slime that 6.0 owners have found floating around in the cooling system (myself included). What exactly causes this and what exactly it is, nobody can scientifically say. There is some speculation that the slime is created by the silicates dropping out of suspension. My one and only problem with that assumption is that the Ford Gold is a LOW silicate formulation (a few teaspoons in the whole 7 1/2 gallon capacity cooling system) and the amount of crap I saw in my cooling system (my first flush) was much more substantial than a few teaspoons worth.
There was one person in the recent past that cut open his clogged oil cooler and tested the substance he found clogging the passages. (I'll be damned if I can find the thread, and do not recall what the substance actually was..grrrrrr)
He found that what had clogged his oil cooler was a chemical reaction related to high heat exposure to a chemical in the composition of the coolant. To me, that says that the additives to the coolant had depleted, and since the coolant was still subjected to the high heat generated in the egr cooler (egt's of over 1000°F is easily obtained), this is what created the gunk. I really wish I had kept the link so I could state this whole thing more clearly.
One thing you need to be aware of is that the 7.3 and the 6.0 differ greatly in the cooling system components. Whether or not the Ford Gold is good for that engine, I do not know. I do know that there are some coolants that can not be used in the 7.3 because of the differing components.