Best polished means that (with the coming of WBP) all 4 Soviet tiers are viable.
I... guess?
I still wager Ostheer T4's design is perfectly viable, it's the panther being left in the dust. The panzerwerfer is a crucial tool for clearing ATG walls and punishing AT infantry blobs. The function, design, and mechanics of Ostheer T4 is all there.
Soviet T1 'lacks' viability comparatively due to T2 having both Maxim and ZiS. Soviet T1 already had extensive existing viability with Penal & Guard synergy. In a sense, all Soviet tiers are viable before WBP.