I dont think it is so much the shortcomings of CFX. As it is technical adversity (maybe?). For instance, I went in and set-up an eyefinity profile, and (just to test the theory) linked my Panasonice 55" Plasma (which is what I usually game on) and my 21" Flat Screen monitor. Then ran Battlefield 3 in CFX on Ultra and somewhere in the 5200's area resolution, and it works beautifully. Didn't play worth a crap, not a performance issue but because with only two monitors you are forced to try to aim where the screens meet. The game played perfectly though, and looked perfect as well. On the other hand, I have several other games that are AMD optimized, like Dirt 3 and Deus Ex:Human Revolution, that run just fine on my tv in CFX at full maximum settings. Also some, like Skyrim, that run perfectly with all settings on maximum with CFX disabled that run like crap in CFX. (Skyrims launcher actually preferred that I run High settings with CFX on, but defaults to Ultra with CFX disabled).
So, I think that as more games come out that are AMD optimized as well as Nvidia, then we will be seeing much better performance out of these 6xxx series cards in CFX. Im hoping to see Mass Effect 3 be optimized for both. Its all just a theory and I could be wrong but there it is.