Lake Balaton

[Kerensky]

In single scenario, there's way too much Russian armor, and too much of it is high quality. Something like 6 IS-2s and 0 T-34/85s.
Also complete air superiority makes this scenario a serious pain for the Germans, all of their infantry and artillery get smashed between the air force and abundant armor.

[Molve]

Per Wikipedia the Germans actually should have more armor than the Russians. (Before the counter-attack of course)

The Soviets used well-entrenched infantry in multiple lines, and also artillery used as tank traps.

(Not that I want more frustrating slogs of scenarios...)