Panthers, with their ludicrous turbo-chargers, can warp around unsupported. Soviet heavy tanks, which lumber about like sclerotic dinosaurs, require a retinue of babysitters.
Like I say I play both factions. I happily spam warp-panthers. They are, however, the best tank in the game and cheap at the price.
You need to qualify this with "I spam warp-panthers in 4v4". In 1v1 panthers are almost never built and in 2v2 you might have 1 out, 2 if the game is going long. I completely understand that heavy tank spam is an issue in larger team games and would be happy to see something done specifically there.
The IS2 in general is a very good tank and generally balanced. The problem isn't that unit directly, it's the incredibly poor squad AI that gets themselves killed combined with the low model count of the Ostheer squads.
The solution has also been talked about to death in other threads. I'm not sure anything else needs to be talked about until Relic:
1.) Fixes squad AI so that it doesn't stand on top of itself so much.
If problem persists (or just generally this would be good I think):
2.) Increase both pgren and gren to 5 man with a corresponding decrease in armor / dps and an increase in RoF on the sov sniper.