Underscores, hyphens, and periods are not allowed in the username.

[lewisgoddard]

I cannot create a new user with an underscore in the username using this plug, but I can with the installer.
The command adduser conforms to IEEE Standard 1003.1-2001. This allows for periods, hypens (not at the start), underscores, and @, and the possibility of adding a $ at the end (samba support).