Don't have the MCU pro but the pro is just the MCU with USB so I'll add my input anyway.
You'll be hard pushed to find a better surface protocol than the Mackie one, used by the MCU, BCF 2000 and others. There is an overlay and for general Sonar control it is excellent overall, however.............
At the present time with the exception of the EQ the PC is out of control reach. There are a few other functions that have gone awry since X1 but they are some of the more obscure. See
this post for info on what doesn't work, although a couple of those have never worked.
I don't use soft synths much but AFAIK they will also be out of the MCUs reach because it doesn't support ACT, so anything you'd normally control via ACT won't work.
FWIW I use a MCU + XT and also have a BCF I use in ACT mode as well as some ACT control on my ZED-R16. I can control just about everything from that combination although a MCU & BCF would also do the job just as well. I can't comment on the C4 because I don't have one, and I've seen several comment that the BCR isn't that great with ACT but I personally find the BCF pretty good, any limitations I run across are more of a limitation with ACT preset surface, rather than the physical surface.
All plugins (that are ACT compatible) and in the FX box are controllable via the MCU.
I could also be wrong about the softsynths, and will stand corrected if someone says you can, but as I said I don't use them much so don't know.
I don't want to sound too down on the MCU because I think it is great and would definitely buy another. I've also had a little feedback from CW that control surfaces such as the Mackie is one area that will be seeing improvements in the future.
If the MCU is a bit pricey to find out whether you're going to like it or not a BCF 2000 has "90% of the ability of an MCU at 10% of the price". I used one for a fair time in mackie mode and that's a great piece of kit as well. In fact there is no better value surface out there IMHO. (Note I said value, not the best there is). See
the guide I wrote for an idea on its capabilities.
Hope that helps a little.