Yes, as soon as I had the 1D X, I was already wishing for more. Sometimes I think the AF system is fine-tuned for decent lighting, and I have felt that it can miss some photos in worse lighting by being too good at trying to track the object, whereas a simpler camera might actually have gotten the shot in focus, not because it was smarter, but because it was dumber. The super advanced tracking algorithms don't seem to work as well in lower light. Yes, I have tried reducing the frame rate from 12 fps down to 8-10. It doesn't seem to make much of a difference. Sometimes I subjectively think it even hurts to change the frame rate below 12 fps (no scientific way I know of to measure it, though). It may be the math formulas that calculate focus are pre-optimized for the exact mirror bounce, delays, resonance and counter-resonance, and other physical issues that happen while shooting at 12 fps. I'm probably way off, though. I was just expecting much better results at a lot lower frame speed (since the AF sensor would be exposed to the image for a much greater percent of the time), and my results weren't really any better.
Another problem I have with my 1D X is that initial focus point acquisition in AI Servo seems to be imprecise. In 61 point mode, I was hoping to be able to acquire focus even on a subject included in only the center point (like the manual says that one can do) and then have that subject tracked using all 61 points.
Instead, the initial focus shifts to nearer objects if the desired subject is covered by only the center focus point. If the desired subject was already locked on when it was more isolated, then it's fine.
So basically it's almost impossible to shoot someone whose head is above a group of players, for instance, because it will lock on the other players all the time, unless I was tracking that player to start with.
If I use single point mode or four-point expansion, then I don't have problems, except I lose the benefit of the 61 point AF tracking system.
I hope that I am being stupid and overlooking something obvious, but I have tried every single possible AF combination, especially the obvious ones like disabling automatic AF point switching, and even the non-obvious ones like the advanced color tracking system, etc.
My 7D cameras seem to have absolutely no problem with acquiring initial focus on whatever is in the center point, even when they are in 19-point AF mode and lots of other objects are in the surrounding AF points.