Its not really comparing apples with oranges because the Eg-S allows you to see in the viewfinder more accurate DOF - even when autofocusing - than the 5D3's stock screen which is not user replaceable. 5D3's fixed focus screen only shows about f/2.8 DOF while the Eg-S on the 6D will come close to the DOF of f/1.2.
So, the fact that the 6D allows the user to swap to Eg-S means that what you see in the viewfinder when both AF *and* MF will more accurately represent the end product. If you do choose to MF to that end product, it is worlds easier on the 6D using Eg-S vs the 5D3's fixed screen.
Yes, the 5D3 will likely allow to set up your shots faster, but that does not make the 6D AF useless.
Indeed. The lack of replaceable focus screens is one of the annoyances of the 5D3 (cost being the primary annoyance). I can live with it, however. When I have the luxury of setting up my shot, I generally manually focus with live view.
I don't think anyone has said the 6D AF is useless. Thousands of photographers have used it for years with the 5D2 (no, they aren't identical, but for all intents and purposes one uses them the same way).
No, it's not useless. But it isn't even remotely like the 1Dx unit, or the almost 1Dx unit in the 5D3. Even in one-shot (which I use exclusively with my 5D2 and much of the time with my 5D3), having reasonably accurate points covering about 2/3 of the width of the frame allows for thin DOF AF with a much higher hit rate. And when you consider tracking, it's apples and paperclips.
I feel like the back button focusing isn't as reliable -or maybe I'm doing it wrong. Are you supposed to be holding the backbutton(what I've been doing) or just press it once? I'm shooting in AI Servo.
It's exactly as reliable. Whether you engage AF with the shutter release or the AF-ON button, what follows is identical.
If you're in Servo, you're doing it right: hold it down, including when you release the shutter.