I hope they have separate teams working on Mobile and Switch versions already. No real need to do this sequentially as they want to use Unity for all platforms, so the platform specific features can already be worked on while the console team is ironing out PS4/XB1 specific bugs. If this is not the case, then they're lacking manpower.
My main concern is the performance hit the Unity engine will incur (code is converted from C# to C++ by software before it is compiled into an executable on console platforms afaik, and maybe there's other overhead). This might not be a big issue for PS4/XB1 (except for the taxing splitscreen mode, maybe), but will be an issue for Mobile and Switch... These are devices with portable hardware and thus will require extra optimizations.