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