How do they behave in AF-S? And in AF-C with shutter priority? Not sure if that can be done with BBF
As stated BC, as a rule I use AF-C and back button focus in focus priority mode, I never use release mode, I tried it and sure enough it releases the shutter to fire even when poorly focused......frankly I see it as a "paparasse mode" ie. you get "a" shot regardless!...........Steve Perry a well know nature photog and Nikon author and expert on AF uses it with BIF, however, then he lives in Florida and there the birds want to sit in your lap they're so tame.
One thing I do find annoying is the D750's shutter button, it's way too sensitive, to the point where when you want to wake up the camera it's difficult not to take a shot, the net result is I don't want to go near that button unless I want the shot.
The D5/D500 has changed the way AF can be called upon in so many different ways without taking your eye away from the viewfinder, AF-C)
BBF (AF ON button)-------activates single point focus.
PV button (front)-----------activates 3D focus.
FN1 button (front)----------activates group focus.
Center joystick button------auto area mode......this is a huge advantage, when shooting BIF.
It's a slick act and makes other methods of AF selection seem like someone has gone out of their way to make it as awkward as possible. I believe the D850 has the same system.
On the D750 with the Tammy G2 you have to change your left hand grip, find the AF button by the lens mount, scroll the thumb-wheel, re-change your left hand grip........then shoot! .....hardly on the fly!