What is the historical argument for this?
Based on some limited testing there are a few good things:
1 Onin War and the mortar upgrade are mutually exclusive. I believe sending the mortars card reduces Samurai siege by 33% if Onin War is there, making OW a net +0.
2 the Mapuche siege attack upgrade (+50% for Hand Infantry) doesnāt seem to do anything to the mortar attack.
3 maxing out your samurai requires 3 age III cards, which is a big ask in the tight Japanese deck.
Itās going to be obnoxious, but it could have been much worse. So there is a bright side after all.
The problem is team games. In fact, the card is a team card.
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Interesting. But I wouldnāt say āItās not quite as OP as it could beā is a ābright sideā. What this really means is that when I tested this I wasted one card slot on Onin Wars and itās easier than I thought to get to OP status.
What 3 age 3 cards? I only see two (the speed boost and mortar).
Any casual mode will be a problem.
It did exist, was used and Samurai did use them.
However the issue is purely gameplay. I still the whole handheld mortar/cannon thing would be better as a Grenadier type of unit and leave the Samurai how they were.
It would also be nice if the Japanese got a Mantlet unit as well, where they could get either the Kaidate or the Sakamogi which will feature a gunner.

Itās a bright side because I thought it would be the most OP thing currently in the game.
In age 3 you have the mortars, Sword Saint (speed, cost, attack and hp) and Warring States (I believe thatās the name) for +15% attack and hp on Samurai and Daimyos and the Shogun.
I suppose you didnāt max them outā¦
Ah, I forgot the daimyo card buffed them too. I actually did send it tho
And is a nod to the Odzutsu unit in AoE 4ā¦