Nikon has a huge back compatibility problem due to the haphazard ,mechanical stuff in their legacy F-lenses.
Overall differences in strategy:
* Sony went from 1 SLR mount (A) to 1 mirrorless mount (E) for both FF and APS-C
* Nikon goes from 1 SLR-mount (F) for both FX and DX) to 1 mirrorless mount (Z) for FX and DX
* Canon will go from 2 SLR mount versions [EF, EF-S] to 2 mirrorless mounts EF-M and "EF-X"
I prefer Canon's strategy, choosing the best mount for each sensor format/image circle. It allows for both:
a) very compact crop cameras and lenses
b) and for great FF cameras and lenses - in sizes from S to XL
People will complain it lacks half a stop latitude that Sony has, that it does take FD lenses, that it does not have auto-focus-bracketing and that it is not Sony.
They will also watch while Canon maintains sales and say it is because it is only because people have legacy lenses, even those for whom this is their first camera.