Discrepancy in lore (spoilers)

[Brightamethyst]

At all the castrums, including Meridianum where we did in fact use them to shoot down airships.

[Jandor]

Garlemald's "air superiority" is typically showed through their use of super-dreadnoughts like the Agrius and Gration - but those have both been dealt with. Outside of those (impractically) massive vessels, it's reliant on smaller escort craft or personal flying machines, both of which can be (and have been) dealt with using relatively mundane means (magic and artillery fire).

The backbone of Imperial air power is the aurora-class airship, every Legatus has at least one and they're significantly larger than the small escort craft. I don't recall any instance of us dealing with those.

That bit about how expensive it is to build, maintain, and operate dreadnoughts is why it rankles the IVth has one. I can believe they'd have one (though I find it perplexing we weren't informed of it beforehand, given we knew about the Gration well ahead of time), but just using the thing should be massively taxing and not at all something to be made into a diversion (as it's implied Gabranth knows Bozja is a lost cause and is just throwing resources there to keep them occupied while he solidifies a hold on Dalmasca). To a fighting force strapped for resources, an airship like that should be used for a last stand or just nonfunctional due to a lack of fuel...

Seems a bit unfair to assume the Eorzeans can make massive progress in terms of anti-air weaponry, but that the Garleans have just stood still on improving their airships, especially as the IVth has been investigating ancient machina from Goug and there are rumours they've found a cache of Allagan knowledge.
Could have conducted research into using a different source of fuel, or massively increased their airships fuel efficiency, or even both. That airship fleet might be near enough free to deploy for all we know.

Yeah, I'm not really buying it either, but fair's fair.

[Wyssahtyn]

At all the castrums, including Meridianum where we did in fact use them to shoot down airships.

Oh right. Those small cannons. That all share the same model, even if they're labeled "anti air", which I expect is more to point out to players "hey, use this thing to progress" rather than as an indication of specialised role. The cannons in question are more typically used as mortars as well, isn't Meridianum the only instance when they're used to shoot down an airship?

[Vyrerus]

Eorzea is weak to flying types

Yeah, it would have been nice to see it directly, but just like stuff like say... Aurora class airships, we don't get to see it. Speaking of those, though, they are said to be, "mid" sized. So between assault ship and dreadnaught, and honestly, from their artwork, they basically look to be half the size of a dreadnaught. Woof. Talk about still impractical.

Though the number of times we fight aerial units or blow them out of the sky is pretty high. Like that time we blew the Garlean Assault Airship out of the sky with mere mortar style cannons in CM. Or how about in Baelsar's Wall, where we blow up about half a dozen sky armors and a dropship without breaking a sweat?

Also, in case you missed it, mages don't need to even be airborne to take out airships. While I'm sure it would help, the thaumaturge limit break calls down cosmological bodies, meteors and comets, to blow up whatever the mage targets. In gameplay it's restricted to the orange/purple targetting system. In reality, it could be used to devastating effect, particularly on large, fortress style aircraft.

As far as the scene where Hien shows up to save the day goes... you're still ignoring the EASE with which the airforce was ultimately dispatched. While I would have appreciated the alliance being intelligent and having a squad of DRGs ready to just Spineshatter Dive every Sky Armor pilot, the scene is ultimately setup to be fodder for the Doman forces and Hien to arrive like sky cavalry. To so easily brush aside the fact that a single dude on a bird with a sword can cut a technological marvel out of the sky with a single stroke is pretty dumb. Also the Eorzean contingent wasn't given the chance to fight back against the air, so sudden was Hien's arrival. It'd just be a matter of ordering the mages and cannons to stop firing on Ala Mhigo's gate, and turn their attention skyward.

[Bhearil]

Are you really using a Hien as a standard of Eorzea's soldiers can do? He managed to hold off (with some help) a possessed Zenos that was stronger than his earlier aparitions until Wol arrived when normal Zenos was shown plowing through normal soldiers like butter while not even trying since his first aparition. Eorzea do have some exceptional individuals but they are far and between. That the Wol can pull off some crazy stuns doesnt mean that normal soldiers can manage even a tenth of it

"Q. How strong is the Warrior of light as a human? A. The strength is as follows: Guts > Lv70 WoL > Zenos > Raubahn > Lv50 WoL. The lv70 WoL beating Zenos was a miracle."

At the very least Hien should be on the level of Raubahn, someone who was considered an invincible legend and that even despite losing an arm can still kick massive ammounts of ass and is a one (armed)man army

Normal Dragoons are not even close to the level of Estinien considering that hes probably the WoL strongest ally looking at his crazy feats, so dont expect a normal Dragoon oneshooting ships (plus they are specialized on fighting dragons, only the WoL its stated to have a personal multi purpose style)

Story has stated that Garleans were kicked all around until they developed magiteck then defeated everyone near (proving that usually magitech>magic sans exceptions) until they started using conscripts to fix their lack of magic issues, so they have a huge edge in numbers and tech

The reason why Garlean empire didnt conquered Eorzea is basically plot armor (designed to create chaos, having a civil war etc), since every time Garleans solved an issue, another popped out forcing them to not focus on Eorzea once again

[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.

[Cilia]

Eorzea is helped by plot armor but there are viable reasons invasions have failed.

First, since the VIIth had the then-newly reborn Alliance dead to rights at Cartenau, it's expanded its arsenal, trained, and adapted to Imperial tactics. Originally the Alliance was just the 1.x city-states: Limsa Lominsa, Gridania, and Ul'dah. They managed to scrape out a win against the XIVth alone thanks to tactical utilization of its forces to the fullest and exploiting weaknesses in Imperial technology (and a teensy bit of help from Hydaelyn, but then again Gaius indirectly had help from Zodiark so it evens out). The next major clash with the Empire came in the Stormblood conflict, after adding Ishgard's considerable arsenal of anti-air weapons and tactics to its repertoire. Finally the hypothetical war with the Empire that played out in the original timeline had both Ala Mhigo and Doma in the mix; neither want for martial prowess.

The Empire's technology can only do so much. It was able to blitz and conquer a wide swathe of territory, but other nations have wised up to how to counter it's technology and strategies and begun to band together to resist (or outright defy) its iron fist.

Second, the Empire's wide swathe of provincial territories is itself a handicap: because it generally mistreats conquered peoples, the Empire needs to keep its legions garrisoned in the provinces to discourage and quash rebellions. That means no matter how powerful its military might be it can't commit all its forces to an offensive without taking a huge risk. If it could I have little doubt Eorzea would fall like dominoes, but the Empire's provincial territories and how they're (generally) managed mean it can't do an all-out offense; meanwhile the Eorzeans are fighting just on one front (Ghimlyt) with all their forces (minus primal-slaying special ops, presumably).

We've never seen an Aurora-class cruiser deployed by the Empire; the only one we've seen at all is Gaius standing on the bridge of his during the End of an Era FMV. Considering each legion's legatus has one as a flagship, I assume they keep them in reserve as symbols of power and authority. The Agrius-class has an abysmal combat record. Nobody knows where the kupo the IVth got that airship or what it can do (but it must needs be there for XII reference). The biggest other airships we've seen are smaller escort cruisers... which can be shot down with artillery fire, as we do during Castrum Meridianum.

For all its power, technology, and advantages, the Garlean Empire is not invincible.

[Brightamethyst]

Eorzea isSecond, the Empire's wide swathe of provincial territories is itself a handicap: because it generally mistreats conquered peoples, the Empire needs to keep its legions garrisoned in the provinces to discourage and quash rebellions. That means no matter how powerful its military might be it can't commit all its forces to an offensive without taking a huge risk.

Which was the entire reason SB worked. Stirring up trouble on two fronts, distracting Zenos and making him further split his forces.

[Damascar]

Those massive airship also require a lot of resources to move around. It's why we created that wall in the burn and why taking back Rabanastre is so important - they need a lot of ceruleum to function and we're basically cutting the provision lines.
In eorzea, we never had all empire divisions against us, but a couple of them at the time, that's why the united forces of the city states, and now ishgard and doma, managed to fend them off.
Also the empire is more fighting between themselves in a civil war that fightning eorzea

[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