Well...the Ascians are powerful, yes - but we're still waiting for them to get a solid victory that doesn't involve the WoL or Cid plot device overcoming all odds and miraculously beating them back. Final Fantasy games have a long history of antagonists who absolutely thrash the main character and their allies - so we really need more of that to better paint them as a threat and because it's so much more interesting than just having them put down at every turn.