It's a common refrain on head fi that more drivers do not make a better CIEM. With some qualifications I don't believe that's true. There's a reason the top rated IEMs and CIEMs have 4 or more drivers. At the same time once manufacturers hit 5 or 6 BA drivers there appears to be a law of diminishing returns.
All things being equal (tuning, crossovers etc...) 4+ drivers appear to generally (note that qualifier) yield better results.
+1 - I just have to think that when you are working with 4+ drivers, the art of tuning becomes much more complicated - like my favorite "Three Stooges" episodes - when Curly pushes one drawer flush, another pops out and hits him in the face. There are a lot of "moving parts" so to speak - so the work becomes more difficult - the challenges increasing exponentially. That's why there is usually a driving force of one individual behind these monsters - someone who loves to tinker with sound.