The Cardassians have been shown to be very weak, militarily. Benjamin Maxwell seemed likely to cripple the Union on his own. Since Cardassians don't believe in battlefield ethics or have war crimes, I assume that Nechayev's comment about war with the Cardassians costing millions of Federation lives referred to Cardassians being willing to obliterate civilian populations with sneak attacks, not actually posing a threat to the military.