Bear in mind that the Empire is consciously designed to promote and extend conflict, not smash everything in one strike. It's why Zenos was made regent of Ala Mhigo AND Doma, why they had in-fighting amongst the Legions when Nael/Eula planned to drop Dalamud on Eorzea, why Solus himself promoted said in-fighting and <kupo> waving contests between Legati and his own Imperial Family. Hell, when Varis ascended the Ascians went into overdrive because he MIGHT have actually crushed everyone without causing a Calamity, which was against the plan. Basically, the Empire wasn't MEANT to completely own civilizations, just to give their populace the IDEA that they could because of "Garlean Superiority" and breed conflict.

Plus, there's the issue of Primals. One Primal on the scene and suddenly half the Garlean troops turn on their fellows for the Glory of Garuda. This was also by design, thanks to Solus (again) making propaganda tactics against the "Beast Tribes" while the other Ascians work with them to push them to summon their Primals. The garleans poke them, the kobolds summon, chaos ensues, rinse and repeat. Even the Ultima Weapon was Lahabrea playing Gaius like a fiddle to rush the next Calamity.

Now, onto technology levels, while the Empire IS technologically superior, they're not "space age" (especially not by ARR), while Eorzea is pre-Industrial Revolution at worst. The Battle of Carteneau proved that Imperials were also fighting with swords and spears themselves, because that's their grunts. Their heavy Machinery is costly, and mostly relegated to the Castrums (hence why they had to actively lure out a Magiteck Armor out of Castrum Centri). The Agrius was also very important because it was the flagship, and first of its kind... and it got one-shotted by Midgarsormr in a not-quite-suicide attack. While Ysayle forced their second flagship into an emergency docking and probably would have knocked it out of the sky if she hadn't forgotten the Z axis.

The Ifrit in Bozja? Yeah, Idk.