Soviet mechs are available in great quantity and have respectable quality for the price. Parts and units are the cheapest to procure. While making some sacrifices in terms of quality, there is usually a significant payoff in terms of certain stats being better than average.
The biggest disadvantage of Soviet mechs is that they tend not to be built with crew comfort or safety in mind. Crews receive more severe injury in Soviets mechs than any others. Stock plenty of med kits!
The simple construction or Russian mechs is a boon. As with the US, repair kits do more good on Russian vehicles. The Red mechs are robust, and will take a pounding, but bounce back easily. On the other hand, they rarely accommodate extensive upgrades such as infrared, radar, air purifiers, etc... Rough and ready, what rolls out the factory heads right to the front.
Another disadvantage is sighting. The optics in Russian mechs tend to be less than spectacular. Making them difficult to aim, and directing strategy towards the close and personal approach. Not a bad thing, considering their above average melee abilities.
A line of WIS-2 "Proletarian" Heavy mechs under construction at a Soviet factory. By the time of the Proletarian's construction, the Red Army was already nigh unstoppable.
The WIS-3 Kommisar was the heaviest mech fielded by the Red Army in significant numbers. Present for the fall of Berlin, the Kommisar had one foot towards Cold War mech design.