While still in the Unicode realm, some people have names that can only be spelled in their native script vertically. For instance people with Manchu names. This case breaks Wikipedia's formatting in comical ways.
Only around half of all languages even have a writing system.
languages with logo graphic writing systems, such as Chinese and Japanese. I think for governmental purposes there is a set of allowed characters, but people may consider the true form of their name to be written with a rare character that's not in unicode.