I'll just give a quick summary, so as not to create any panic here. I believe the reason the 5D, 5DII, and 7D acquired focus lock on the low contrast, low light, "stationary" towel in AI Servo was because they have a slower frame rate (the 7D lowers its frame rate automatically in low light) which seems to put the emphasis on focus acquisition. The 7D, MKIV, 1DX also have the default feature for first Image focus priority, which also places an emphasis on slower focusing, as opposed to shutter release. We're not yet saying that the 1DX doesn't track well in AI Servo, we're just saying that the first focus lock of a stationary or slow moving subject in low light, low contrast situations is difficult to lock-on when higher than 10fps are in use.
Keep in mind though that when the frame rates of the MKIV and 1DX were lowered they did perform better than the 7D in my testing. I don't own the 5D or 5DII, so I'm going by others findings with regards to this. In case people didn't read the whole thread, at least in my test, these findings were occurring in low light, low contrast situations of a stationary object in AI Servo in order to lock on the target prior to it moving, to establish focus lock so that the camera will be ready to begin tracking once the subject started to move. Sycotek wanted to test the threshold limits of the 1DX's ability.
"I will say though, that in my experience in real world shooting with the MKIV, in low light, low contrast situations, and shooting at 10fps, I have never had a problem in AI Servo mode, and the 1Dx seemed to perform similarly under the testing conditions above, however, I haven't had a chance to test it in game situations yet.
Recommendations would be if the MKIV or 1DX couldn't lock on the first frame of a tracking burst in AI servo, then the shooter needs to make sure that they grab an edge of the subject to create more contrast (I do believe that this is common knowledge at this level) for focus lock, once the subject starts "moving" and the distance to the camera increases or decreases rapidly then this is supposed to aid the camera in AI Servo tracking. The other point here, would be to lower the fps to less than 10fps if a problem did arise in real world shooting situations.
I think a good analogy would be if you had 2 race cars going through a slalom course, lets say that one car was limited to 50mph (7D) and the other car was limited to 100mph (1DX). The drivers were told to make it through the course and around the cones as fast as they can without hitting any cones. The 50mph car would have the same chance as not hitting any cones as the 100mph car, because it's up to the drivers to not exceed the limits of the cars. Just because the one driver can go 100mph, doesn't mean he should. He also has the option of going 50mph, or he can go 75mph and win the challenge, or he go 100mph and drive off of the course.
Viggo, in further response to your concern, I don't think that this is as alarming as you think, because the 1DX can be slowed down to meet the needs of the shooting conditions, just because the 5D, 5DII, and 7D are already slowed down by default doesn't necessarily make them better. I would also be curious to see how the 5DIII behaved, however, I expect that it would behave similarly to the 1DX would if we were to reduce the fps of the 1DX to match the frame rate of the 5DIII.
We also have to be careful when analyzing focus points on the monitor or LCD, because if the camera were to miss focus on the center low contrast point of a subject while holding down the focus button, and then we were to re-compose the shot to the edge of the target, the focus points may look like they were focusing on the higher contrast edge the entire time.