While overall I'm still adjusting to the full frame Nikon after using almost exclusively the Fuji for the last year, yesterday it was the first time when I was really impressed with the D750.
Went outside with the kids after 8 pm (long into the dark for Michigan) and the only light sources were some lights from inside the house (very weak, inconsistent) and the occasional flashlight the kids were using from time to time. I did activate the AF light on the 750 (I usually deactivate it for most situations). Also I was using the 5 years or so old sb-600 flash.
All in all I couldn't believe the focus. It was common the hear the kids, point the camera in that direction, couldn't see a thing in the viewfinder but I was pressing the shutter anyway and when I was reviewing the pictures they were in focus! I will repeat I could not see them in the viewfinder (the night vision was minimally affected by the lights in the house). Yes, I could distinguish them without the camera to my eye but it was dark.
I was using the 24-120 @ F4, exposure time around 1/160 and the autoISO was going around 3200, AF-S with group area, usually the center as I couldn't really see where to place the focus.
I am impressed!
PS. Now I have to be negative and say that for better light situations the focus could be more accurate, at least compared the Fuji X-T1. But part of it probably can be improved with me learning more how it works.