The weapons for the basic infantry of each of the factions express very well, where they expect to be fighting and their personal combat doctrine:
The Polonian Rifle is the simplest weapon, and has the longest range. Perfect for a country which has limited heavy industry, a primarily rural population, who are more likely to be at vaguely familiar with its operation and is expecting to be fighting in the forests of their own border regions.
Saxony equips their men with Submachine guns and full body armour. These weapons likely take many factories to build all the different parts for them, but Saxony is one of the most industrialized countries and has the industry to support this. They also are expecting to engage their enemies in a wide variety of terrains, from compact cities, to open fields. As such the Saxony infantry weapon is designed to be an effective weapon even in these disparate terrains.
The Rusviet trenchgun is an effective example of their overall mentality. The Vanguard are not equipped with armour like Saxony or long range like Polania. Instead the Trench Gun requires the use of a rush of manpower to overwhelm the enemy and then blast them at close range. It employs a rush of inexpensive units and uses powerful weapons at close range to destroy their opponents. Leveraging the size and might of their state to win.
Usonia's automatic rifles display both the massive industrial power of Usonia, and the scientific capabilities of the nation. The weapon is more complex than the Polanian rifle or Rusviet trenchgun, but harder hitting than the Saxonian machine pistol. It also shows the logistical strength of Usonia; where Saxony can ship their parts and replacements along Europan rail lines, the Usonians need to send their equipment across oceans.
Polania's riflemen are actually a shared unit. When the game needs to throw partisans at you, it sends Riflemen.
Which would suggest that Polania, the country which has; generals running off with their own personal armies, has frequent border incursions, logistical problems, has a government struggling to protect its territory and uses massively outdated technology, is one where the army is made out of just that. Largely mobs and partisans. If your going to be equipping a large force made up of everything from former farmers and rural workers, to partisans, you likely will favour a weapon which is simple to use, simple to make and most of your forces are likely at least partially familiar with. Hence a rifle.
Mechs as a whole don't make a lot of sense when real-world strategic and tactical factors are taken into consideration; Many mechs are way too tall for their own good (excessive height makes it a lot easier for the enemy to spot a vehicle and hit them with a long-range anti-armor weapon), leg systems are a glaring vulnerability (little armor and being an essential structural point) and all the jittering and bobbing around the in-game mechs do would throw their aim off terribly. But then you remember that these mechs where initially invented by Tesla who distributed the technology with peaceful intentions to the major European powers. Suddenly the reason the "Śmiały" is carrying an upscaled bolt-sction rifle makes sense; it was likely a civilian design hastily weaponized after the major powers started eyeing each other off - likewise for the the Arm Cannon of the "Grimbart".
This also begins to explain why there wasn't much apparent innovation during the war; the major nations fighting likely knew how to manufacture the mechs but didn't understand the underlying technological principals well enough to experiment with them.
This may also explain Saxony's "walking tank" designs. Saxony, being the most industrialized power fighting in the war, likely has the most ability to experiment and test different mech designs and the angular appearance of the "Isegrim" likely represents the application of the "sloped armor" principal (Something the IRL Germans where struggling with during WW2). Bizarrely enough this may result in Saxony's Armored Fighting Vehicles undergoing a sort of Convergent Evolution into real-world tanks.
