I suppose that we can't use modifier key + insert, start and such keys is by design, so ok, though personally I'd like to be able to use them as those keys I never use within any game or application. But hey, if using them can be a source of issues and it's decided not to accept, so be it.
A potential issue, maybe it isn't? We can't use international keyboard keys. I mean, if I go to change a shortcut and press Ctrl + Ñ it doesn't change it. Modifier key with numpad symbols (+, -, *, /) don't set the shortcuts, either. These I don't see why should be rejected.
And what is definitely an issue is using modifier key + num pad number. If I press Ctrl + NUM_PAD_5, the shortcut is set to "Ctrl + 5", but then it won't work. Neither using the 5 from the numpad or from the main group. If it accepts the change then it should work.