Takeuchi: Most people don't seem to think of her as a Servant.

Nasu: Medea wasn't exactly what you'd consider an "impressive" Heroic Spirit, so that's probably why. Still, from the perspectives of mages in the present era, she is quite godly. She can easily cast powerful spells that would require at least a month's worth of preparation for normal mages to cast. We don't have her casting anything as outrageously powerful as a nuke, though.

Fate/complete material II; Characters: Caster, pg. 84-87