I would imagine they would be a faction that has nice defensive infantry but terrible armor. Maybe instead they would have to rely heavily on off-map abilities, similar to the close-air support and ostruppen doctrines. From what I can understand, they are a WW1 army with state of the art naval and air support.
That's a bit much. Japenese land forces where much better then WW1. It's just that their land forces production never really exceeded what the allies had in 1942. Their weapon development was compararable to allies too maybe a bit worse. Their biggest problem was more that none of those prototypes could be built do to lack of resources and what was was spent on naval and air vehicles.
To be honest, a lot of WW2 propaganda still effects a lot of peoples opinion of the Japenese army of WW2 at the time leading lots of people to believe they were more inferior then they were. The Japenese were still very advanced compared to other nations at the time other then germans and the big 3 allies.