Hard to say before release, but:
1. Japan has a big army. Attacking China is will be a gamble (as it should be), but if they fight Soviets in a fair fight, it will be a tough battle.
2. Soviets will take a long time to transport their western armies to Eastern Siberia.
3. It is risky for the Soviets as well. The Poles, Romanians and Finns can take on SU.
4. It seems more benefficial for SU to take care of the nations above first, for strategical reasons.
5. Perhaps most importantly, SU need to take the Great purge, which greatly cripples their army. SU need some time to become scary enough. Until then, Japan should get themselves an ally (if they do not, it is their fault).