The project files are completely platform independent. That said, there are a couple of things to keep in mind.
You need to make sure that, if possible, the inputs and outputs (including their names) of the audio interfaces are set-up the same. Otherwise Cubase will tell you that it can’t connect the input/outputs.
You need to make sure that you have all VST/VSTi plug-ins used installed on both systems. Here’s where a project might become platform dependent; if a plug-in is only available on one of the platforms! If that is the case, simply avoid using that plug-in.
If you follow these “rules” you’ll have no problem moving you Project folder between the Mac and the PC.