That is not at all obvious, Joelatron, and certainly not for the reasons you mention. MSU can be done with units able to act independenty but it can be done with a synergistic CA force. This is no criterion for whether a book is able to field MSU.
It's odd that you would call independants more capable of MSU and back that up with an example of a very synergistic buffing force. Cauldron + Mind Razor + Corsairs or any other combination are a good example of the sum being more than its parts. It tells us very little about the average size of units in an army, their number or how it usually fares, and I can do the same - conceptually, not in the details - with the Empire book.
I don't see how the SoF or Mace made the Empire MSU. People were fielding hordes ever since 8th without these items. Magic items are just details with little bearing on the concept if two armies can put the same concept into practice. It's true, mage buggies make infantry more expensive, they enhance it, they need to work together and they provide better synergy. Again, that says nothing about the viability of MSU. A mage wagon is quite small. More expensive infantry could be fielded in smaller
I really don't see why the new Empire should not be easily able to make both hordes or MSU or anything inbetween. I do think that most players will tend to the third choice with a couple of blocks and detachments + stuff. However, it's entirely possible to use for example 2*5 Reiksguard, 5 or 2*5 Outriders, some WM, knights, 3 or 2*3 DG, archers/huntsmen, some chariots and a couple of 30-strong infantry with 15 -strong detachments. You CAN take the CoC somewhere, you don't have to. You CAN take all kinds of magic items, you don't have to. Magic items do not a MSU force make, and synergy has nothing to do with it either. Special slots have nothing to do with it either because it's completely possible to take many small units from Lords, heroes, core and rare...
I have yet to see a successful 8th book be done MSU due to key units moving to the special slot (that works with the BRB scenerios not just battleline). Tombkings come close, but are very reliant on keeping 1 character alive, so it can all unravell very, very quickly! Ogres dont count because there isnt many of them to begin with, so everything is "MSU" (less models in the army than the average skaven slave unit).
Didn't TK have to keep their general alive in 7th edition/with their old AB as well? I know that Skaven do and VC better did with their old book, and both were usually fielding hordes, so that disclaimer isn't particular to MSU either.
Lastly, I don't see why Ogres do not count. They cannot be considered good MSU because they field small units? Uh...I think you need to explain that a bit more in-depth. Perhaps you meant to say that armies that are inherently good at playing MSU should not be considered...?