Lots of services limit usernames to a certain range of characters, but some don't. However, there's tons of invisible unicode characters that you can use instead, so even without this one, the problem would still exist.
Thеrе arе many cyrillic lеttеrs looking idеntically to latin lеttеrs. In previous sentence all "e" letters were cyrillic, for example. So probably the only sane way is to limit username characters to latin letters.