I didn't look into the d750 specs and I'm not in the market for one, but you do notice that the full-fledged 5d3/1dx af array is only available with some select new and expensive lenses?Yeah, new and expensive lenses like the EF 50mm f/1.8 II and the EF 35mm f/2, both from 1990 and one of them costing just over $100. I guess you didn't look into the 5DIII/1D X specs much, either.
LOL, you've made your point - I can use the full 1dx af array with a 50/1.8 ;-)
My point is: popular lenses like 24-70L1/2.8, 100mm macros/2.8 aren't group A - other popular budget choices like the 400/5.6 have a lot less cross-points which is about on par with the d750. Thus general statements are difficult with the Canon system while Nikon really just depends on the max. aperture up to 11 cross points at f8 (5d3: 1 (one)).
There are over 40 lenses in group A and can utilize all 61 AF points and 41 cross type sensors as well as the 5 dual cross type sensors. These lenses are Canon's performance AF lenses that need the speed and accuracy this system provides. I am not going to use a "budget" lens and complain about the AF performance. Does Nikon even have dual cross-type sensors?