Despite all the tech, garlemalds fancy toys aren't actually as op as they would seem. They require a lot of resources and logistics compared to their magical opponents who just need regular supplies. They are also very vulnerable to magic since the magitek gold mount specifically says they had to develop magic resistance coatings to stop them being blasted silly via even basic spells

A lot of them are very glass cannon esque, or they have structural weaknesses that their researchers aren't fixing as garleans focus more on firepower than defence (As any engineer could tell you, all them walking machines would be crippled easily by smacking the joints, given garlemald is in mountainous territory however walkers are useful for terrain traversal. This is why the hexadrones are deadly, but they are incredibly complex and rare- they are a new development and wouldn't fare well in garlean homeland)
Geomancers messed about with the wind aether and grounded garlemalds airships, hence why doma was invaded through nagxia through a tough campaign. The airships require lots of fuel as well which is why they raised the shield in the burn- to force the garleans to use a fuel heavy route.

The weapons are prototypes. Sapphire weapon required oversoul just to stand a chance at defeating the G-warrior. Emerald had to be snuck out to finish development. Ruby was in its field trials like one of the scorpions in ala mhigo. All of them ended up swiftly destroyed.

Finally, when the airships DO get the advantage, they are exactly as OP as advertised, such as when dalmasca got taken over. Air dropped over nalbina and dalmasca and it was conquered.

The good guys magic is very much a powerful and flexible tool. One mage might not be able to do much besides cast fire 1 or 2, but when you got a bunch of them together as artillery, those fire spells will saturate an area as good as any magitek cannon at no fuel cost or metal cost besides the usual small arms resources.
Finally, the good guys have been teching up as well. Machinistry and ironworks things especially if one of the bozja field notes is any indication. The good guys also have an advantage in that they can find weaknesses in the garlean mechs, but the garleans cant stop magic working on eorzea's side.