Is it 'safe' to use FN for custom shortcuts
You cannot create shortcuts using the fn key in the preferences pane (but you can assign Function keys f1, f2 etc.) 

The shortcuts interface will allow you to press fn + X for Cut etc, but only X is displayed as the shortcut, and pressing fn + X results in typing X not cutting.

Very annoying for people that switch between Windows and Mac keyboards, since the CTRL key on a Windows keyboard is where the fn key is on a Mac keyboads.