IMOP it is more about the players being the difference then the weapons at BR5. I play all BR five factions except the Japanese, and when I lose as a BR5 Soviet I don’t feel I lost because of the weapons, It was usually because people on the German side were building more spawn points, or I just got matched against a stack group. I have just as good of Soviet BR5 squads as I do German or Allied BR5 squads.
Machine guns, the Soviet RD44 MG is decent, has a 100 round drum and its rate of fire is over 800. The German MG42 100 is slightly better, but I would only give a small German advantage, however it is TT as opposed to the RD44 being premium.
Assault Rifles, the Soviet AS-44 and German Stg 44 are fairly even IMOP. I consider this a draw.
Semi auto’s, I actually prefer the Soviet AVT40-20 over the German FG42 II. However I would consider them a draw as well.
SMG’s, I really don’t use many SMG’s in BR5 as I prefer the Assault rifles, however I do have one five man Conders MG squad, one Soviet 1921 Thompson 100 and one PPSH41 Parkerized squad. The Conders and Thompson both come with 100 rounds, a high ROF, but the Thompson has a higher damage rating. The Conders and the PPSH both shoot at 1,150, and the Thompson at 1,040. The other German option would be the Kiraly. IMOP, SMG advantage Soviets.
Tanks, I don’t play with tanks at any level because I find them boring, but everyone has their own opinions and bias, however the only time I would consider blaming a loss on a tank is when they camp in the gray zone while constantly blasting cap and spawn points. 
Planes, both basically have equivalent jets, Me262 vs Su9 (same bomb load), both have equivalent bombers, Ju 188 vs T 2S (same bomb load), so IMOP it’s a draw.
I can only compare weapons I use, however, I never feel like I am at a disadvantage with BR5 Soviet weapons, only at a disadvantage when I get bad team mates.
And yes, I lose far more often playing BR5 Soviets then BR2 or 3, however as I had said, it was because of the players, not the weapons. On the flip side, I rarely win playing BR2 Allies, even though I absolutely love my squads and weapons. Again, bad team mates.