In my experience so far a good Russian player playing conservatively can make Moscow close to impossible to take. The way Russian territories are set up allows them to fall back and shadow any approaching German army until the Russians have reinforced their stack to the point where it can turn around and chase out or destroy the invaders. There’s no real downside to this vs standing and fighting as you’re only giving up 1-2 ipc provinces as you fall back and you’re continuing to preserve and grow your Russian stack at a rate faster than the Germans can hope to reinforce theirs (assuming, of course, that GB and the US are putting any sort of pressure on the other side of Europe). I’ve pretty much given up going for it and instead have resolved to find some way to take Leningrad and Stalingrad. I’ll admit that Germany and Italy, given free reign to attack Russia can eventually bring them down if they focus all their strength on it but how are they doing this while preventing or counter-attacking allied landings? There’s just not enough IPC’s available for them to effectively do both.