I have yet to port my scripts (https://bitbucket.org/znmeb/osjourno)
from 'yum' to 'dnf'. I'm not sure I am going to unless the live ISO
creation tools also switch. But I have tried both 'dnf' and 'yum'
manually during the F21 alpha and beta test phases. I think there were
cases where 'yum' said there were updates and 'dnf' didn't. And it
seemed like 'dnf' was slower.