Discrepancy in lore (spoilers)

[Brightamethyst]

The reason why Garlean empire didnt conquered Eorzea is basically plot armor

True, but Eorzea also has other factors that make it a harder nut to crack than someplace like Doma or Dalmasca. Even Ala Mhigo probably only fell because it was already unstable (and alone, after the Autumn War) when Garlemald invaded. You've got dragons, primals, a city state who's entire army is specifically trained for anti-air tactics, and aether so dense it literally makes people sick until they get used to it (though the writers have been mostly ignoring that last one these days.) That, combined with the fact that the goal is conquest, not annihilation, makes a takeover much harder.

[Bhearil]

Dragons usually wont intervene on mortal issues usually so dont expect them to do much unless their species got threatened by the Empire. Ishgard anti air weaponry is mid range at best since dragons ranged atack is limited in range, meanwhile Garlean ships can bombard from afar carpet bombing style. Ala Migho was on its prime as strong as the three main cities combined, but King Theodric civil war weakened its power making an easy pick for Gaius Van Baelsar. Dalmasca is more of a conquered yet not pacified region with guerrillas roaming around, but they werent a serious threat until they joined forces with Orthard and even so their victories were bloody

The most worrysome issue for Garlean Empire are the primals due to their wild card nature and lack of counters against their tempering

[VargasVermillion]

Another underrated talent that people overlook is garlemald is supposed to be excellent at intelligence and subterfuge, both of which were required to defend their borders prior to magitek. That at the very least gets more attention than their airpower but it's still fairly under-represented.