Totally misleading, there are reasons to use BBF (or not to use it) but that does not change with mirrorless.
The main reason not to use BBF is that it needs substantial training, which made me reluctant first to use it until I was able to automatize it. With my SLRs i usually have individual configuration for slow operatoins and high-speed AF situations. Originally the first was still being set to shutter release Af- activation but in the meantime I am solely relying on BBF for best control. Iam using AF-C only (and if the camera is offering Iam deactivating AF-S completely so i cannot accidentially switch in the wrong mode). With BBF i can quickly control whether the AF is on or not, so I can more easily avoid AF focus hunting caused by obstacles like leaves and such and I can do manual overrides at any time, otherwise the override mode that makes AF-S lenses so advantageous is overriding me again. AF-lock buttons do not work for me, even more so the buttons on the lens i have found to be unreliable.
I am working in single field mode mostly and with PV button configured to switch to group mode. Having the camera to chose AF Field and this 3d Stuff might work for sports but not for birds in flight. Yes i have to switch my thumb between af field choice and AF activation but if I have to be really fast there is no time for carefully chosing the perfect af- field, i either preselect and then hold AF or decide to keep the focussing and adjust composing afterwards. Having a greater AF-field coverage of the frame does not change that for me. If there is enought time (like a sitting owl) there is no problem with BBF either
What is different between SLR and mirrorless? Mirrorless AF fields covering a greater area, and are not on par with af speed and reliability (at least with Nikon Z, dont knowhow evolved Sonys A9 actually is. I see no need to change my habits with the Z6 in terms of BBF/AF-ON button