In Fedora Linux, there is no difference. adduser is a symbolic link to useradd.
$ ls -l /usr/sbin/adduser
lrwxrwxrwx. 1 root root 7 Mar 20 20:00 /usr/sbin/adduser -> useradd
You can use either name, and they’ll do the same thing and act the same.
However, as you’ve noticed, the adduser command does not have exactly the same behavior as adduser from the Debian world (which Ubuntu fits into). So, it might be best to get into the habit of using useradd to reduce possible confusion going back and forth.