To justify such high prices?
They seem to need a fair amount of setup and programming to work with an Axe 3.
The only ‘advantage’ I see is the additional use of color.
What am I missing?
Essentially, what you may be missing is that they are totally different devices that happen to intersect at the Axe FX.
The RJM Mastermind is a midi controller that can work with any midi controlled device, or control many of them independently at once. It will work with
all Axe FX versions. To make best use of it's capabilities, it has the largest screens, per switch, and main screen that I have seen on a midi foot-controller. These screens have control of color, font boldness, contrast, and more than one function per screen. There are 16 pages of these screens. Any switch can control anything, as it can be an on/off, either/or, or control a multitude of functions at once. The fact is that the MMGT is the premier midi-footcontroller in the industry, and to get more you would have to have one custom built at multiples of the cost.
The FC series controllers are task-specific to the Axe FX III. They don't control anything else without the Axe FX III. They are powerfully tailor-made for the Axe FX III and are, quite simply, the easiest plug and play option you will find, period. A great unit, no question, but inexorably tied to the Axe FX III, and it alone (they will not the control previous Axe FX versions).
The analogy I think makes most sense is a standalone computer versus a workstation. A standalone (MMGT) can do everything inside the box, so it can be tied to anything that understands its format. A workstation (FC) is a remote control that derives all of its power and functionality from the server (Axe FX), and can only perform its intended function when connected to it. This is why a standalone computer costs much more, and the same applies to these two very competent foot controllers. They are as different as they are similar, and the differences justify the cost if you need them.