the covenant would massacre the empire no contest. the covenant is capable of destroying entire planets with their normal cruisers. the empires ONLY weapon capable of that is the death star and that screams massive target for covie fighters
the covies have not only air/space superiority but also ground superiority. the only thing that the empire could use to even the playing field would be to pull out some sith but they are so few in number it wouldnt matter.