AAD Usernames need to support all character sets
Many customers allow usernames with special characters, double byte characters and Asian character sets. AAD Connect and Azure AD do not support all of these character sets. Not all customers use Active Directory on premise as their main identity store. Thus identities with special characters cannot be synchronized into AAD. For customers with hundreds of thousands of usernames with special character sets, it is a horrible user experience and very costly to try to rename all these logins.
Rob Brown commented
The most important character is the "+" symbol that should work within the username.