Dude, It's just most people play Japs when they are still conscripts, I mean it's extremely easy to play as and requires no skill. "Japan Lamers" as they are often called can beat people with a much bigger PR level than themselves. In fact, if you check the stats, 80% of players that used japan won with it, most civs have 20%.
Why Japan is OP:
Cards can be sent twice (I would understand if it was weak cards but c'mon 1000 wood,coin, and food twice???)
Consulate is very powerful
- Can age to 5 with no resources just export
- has best consulate civs
- has best consulate soldiers (by far)
- consulate enables units being trained 10 at a time! -but units get trained slower... and you cant get a HC shipment to boost that, but you can get 1 from you dutch allys
Houses generate resources There for does not recieve factorys
Has best wonders in the game
75 villager cap (not only is their army the best but the largest in the game if every civ has max vills - but not as many resources for that reason
can spy without spyes for 2 seconds, every, I think 10 minutes or so
Sioux also have a very OP army (though not as much as Japan's), but to make it even their economy is nothing (they have 1 eco improver card)
Sioux can't make walls or mortars, Japs can