Soviet anti tank grenades are a pretty huge threat, an engine damage tank is a dead tank, and conscripts being able to Oorah and throw the grenade is a huge advantage over other infantry Anti tank.
The key with soviets is to "snare" a vehicle with;
Mines on combat engineer
Guard rifle infantry Button ability(unlocked when you research DP LMG)
At grenades
And then finish with
Mark vehicle ability(increases damage inflicted to a target vehicle)
Zis-3 field gun
SU-85
If the tank you're trying to kill has a healthy engine or isn't marked with mark vehicle (or a combo of both)you're doing it wrong
The soviets just require more combinations to knock down heavier vehicles quickly., unless you have an IS2.
You also mentioned you were playing 2v2 or 3v3?
Make sure you each mix Commander choice into your team strategy to maximize your versatility as a team .(1 guy goes for guards and mark vehicle, the other goes for IS2, and another goes for ISU152,etc)