Normally I tend to write detailed reviews but speaking as one who hasn't read the full Code Geass universe, all I can say is that there are many elements in this side story that makes it feel like a poor rushed adaptation.

Nothing horrifying but in many ways when a work cannot match up to the previous work but tries to be part of that universe, there are just elements that will make the disappoints worse.

However if you're wondering why my personal rating for Nunally is 10 and the overall is 3, this is because I read the scanlated letter of the person who made this during the final chapter and it was just... so heartfelt - I couldn't deny a person who tries to make a masterpiece and succeeds at it short of a 10 because I didn't like said person's vision. On the other hand, reviews are still for those who are "curious" about a work and so I don't feel like lumping my two ratings together especially since you must literally endure up till the end for that last message to have such an impact on your perception of this manga.