The problem is Japan never focused on developing their army, they had good support weapons like mortars and MG's, because they focused on massed infantry warfare, not tank battles or long drawn out sieges. They had good fighter craft and naval ships because that's what they needed.
For the factions in europe, you can give them rarer units to balance things out because said rare units did in fact exist. Short of literally making up units that didn't exist you couldn't put a Japanese army in coh2.
Again, talking about history.
I know the history. And I do not mind the fact that Japan had a bad ground troops.
We're talking about the game. And the fact that Japan had absolutely everything that would be a new, exciting faction.
You are the commander - show how to fight, show tactics.