the best way to counter it is with Stuka. The soft counter is just skip on HMGs and mortars, or get 1 to cover your flaks. Get an aggressive commander, preferably with smoke bombs and just roll over the SOV guy.
As for the unit, imo the popcap should be bigger, to punish spamming. I can't say it is OP, but it is definitely strong.
Well yes and no. I'd say trying to counter 120mm with stuka specifically is a bad idea, since it has long CD and even if you de-crew 120mm soviet will just crew it back. Stuka helps soften soviets defence which might allow you to push and at least force 120mm away.
And 120mm is not OP in a sence of perfomance. Its imo just overtuned for the timing and stats it has. If it was a little bit weaker surviability wise or came like not at 2CP but at 3CP, it might have been completly fine.