Katana in the game? Ehh that doesn't sound realistic because it takes quite some work to make such a sword and I don't think that the people after a post apocalyptic war would still have the knowledge to make it. Not to speak about the fact that its in America and lets be honest how many Americans even now know how to properly make a Katana...
Now offtopic: About Samurai's vs European warriors, look at it from this side the reason why Europeans were so heavily armored was because they moved in formations and acted as almost indestructible walls. Trust me those Samurais would not come close to killing a Spartan Phalanx or Roman testudo formation. Against those arrows didn't work and you had to come close to fight them. And a Spartan spear is always longer then a samurai sword, and I seriously doubt those light weight armors would hold a spear penetration.
Plus on a side node by the time Katana was available Europeans already had basic handguns and canons. And if we look at the overall military efficiency the largest empires were always of European origin. Be it in the pre-christian era, middle ages or even gunpowder era.