Garlean cruisers are pretty impressive whenever they're outside of airspace guarded by ballistae that fire exploding lances the size of houses designed to hit significantly faster and equally sturdy targets.

Even before that, as imposing as they are the opposition they have is people who can will explosions, lightning, and permafrost into existence in the most vulnerable points of their designs. And casted magics are powerful, but often forgotten is magic is in *everything* we do, and in *nothing* they do. That medieval sword and armor? Magic is a part of its very creation process. The strength of your swings with a sword? Empowered by magic. All of it is always magic because it is innate to everyone but the Garleans and it is a massive advantage that the technology they have only brings to equity at best.

There's also the fueling issue which is brought up in Stormblood in a few points. Big ship isn't going to do anything but become grounded if they don't have supply lines set since its fuel and munitions run on the same resource.