I'm implementing a new user authentication system for my mobile app and was constructing my username rules, i.e. less than 40 characters, alpha-numeric, etc.. when I thought about why I should only limit it to alpha numeric.
I don't have any issues from the application view - they can use emojis and foreign characters for all I care. I've been trying to figure out reasons why I shouldn't allow it but couldn't think of any good reasons other than it might be non-intuitive for users to search by username if it's just an emoji. Are there any other reasons or landmines that I'm overlooking? I'm using GraphCool as my backend-as-a-service.