Imo that is an indication that USF and Soviet do not mix well and an explanation for that could be that Penals seem to be more cost efficient than Riflemen or Soviet have better units option for supporting their Penals.
That has hardly anything to do with penals.
Soviets do have reliable infantry that aren't cons.
Soviets do have early game light vehicle, indirect fire, mobile army, good elite infantry in form of guards to replace any previous squad loses, light handheld AT reliable enough to fend off lights(except FHT) and deter rushing in with tanks, reliable med tank, reliable TD.
USF doesn't mix well with soviets, because soviets have all the tools USF brings in and more.
Duel soviets have better synergy then USF+soviets as USF is per unit stronger, but overall more limited soviets.
USF mainline infantry might be strongest with upgrades and weapons, but other then mainline infantry excelent scaling, USF doesn't bring anything to soviet team mate. Maybe team weapon drops through paratrooper doctrine, but UKF can do that too and offers what soviets do not have, complimenting them well.
USF was designed around strong shock units to take early game map presence and all of that shock potential was castrated over last 2 years with constant nerfs to the point their tech structure redesign became mandatory, so I'd be very surprised if they were any popular in 2v2 - they just don't bring anything to the table compared to other 2 factions.
Once patch drops we might see them more, but until then we can only speculate. What is sure is that tech revamp and commander revamp might help then enough in team games for them to become relevant again.