The weakest faction so far is Sturgian, even with e1.3.0's fix on their equipment and state. And they have shield for their tier2 infantry. The balancing problem the game as is that there is no extra attrition in snow area, the speed penalty is not yet notable and the horses don't consume grain. No matter how shielded this faction is, since the AI don't understand how to use a shield wall yet and don't have a tactic AI that can make a good use of heavy skirmisher cav, this faction will keep been slaughtered.
Asarais have a similar condition compares to the Sturgians, their territory is linal that makes their army harder to rally, their desert don't provide campaign-wise bonus for them, and plus, they don't have shield for neither of their tier2 infantry. But they are doing fine in most campaign I've played, probably because they have quite decent shock cav units, and their no shield basic units are decent against cav.
Valandia don't have shield for most their infantry units, but they are also doing fine, that's because they have great shock cav units. It's very common in campaigns I've played to see Valandians conquered settlements from Sturgians, Battarians, and West Empire, they all have more shielded infantry units comparing to Valandians.
Battarian usually last for long enough to see at least one of the empire factions been crushed. Their tier1 is trash compares to the rest cultures, but their skirmisher are really doing fine. Those are very decent skirmisher since tier2. Indeed that their tier2 melee infantry don't always have a shield, but their spear made them less killable. Their archer units are even better, although not in number, but are good enough to make a difference in end battle range duel.
As soon as the game no longer let lords leading army of recruits, you will find out that having a shield for tier2 or really doesn't matter that much. Adding a shield for tier2 is easy, even I can do it in less than 20 minutes, but I wish TW work mroe on battle AI, the AI need to understand different formation group according to the unit's equipment set, and make a better use of each unit with a decent timing. So far the AI seems to be using the same formation group setting as the player's, but I think they need to have their own tactic in the battle.