If you're getting PTRS on your cons then you don't have any call-ins.
SU85, katyushja and T70 are good/decent, T34/76 is useful because it's a tank, albeit a crappy one. Halftrack is situational, SU-76 is worthless.
I'm not gonna argue about the PTRS, I was just pointing out that late game blobbing is finally viable for Soviets since they get an upgrade.
About your next point, I don't simply mean to compare units. For example, SU85 is pretty good, specially in open maps without too many shot blockers, but that's not the problem. The problem is the performance of a tier as a whole. If you decide to go, let's say T3, your hands are tied with units which don't scale well and without call-ins you'll lose against a decent player. If you go T4, you'll have no breakthrough units and are forced to play a little passive. With T3 at least you can swarm the enemy with T34/76s.
The other factions don't have this problem as the Soviets do since they have better stock units.