Doesn't this apply to a lot of soviet doctrines? Are they op too? Or is it op only because it's Axis?
Soviets doctrines don't provide mid-game vehicles. You have to rely on ZiS which is very risky some times.
In this situation you have Stug E which can fill this gap.
What's more, there is not any doctrine so useful on all stages of the game. Some doctrines have 2 early games abilities and 3 late game, others have 4 early game and 1 late game.
Here you have everything. Early game ability, mid game abilities and late game ability.
Skipping PzIV is not so risky when you have Paks and Stug E.
Skipping T34 and SU85 and waiting for IS2 or ISU may be very, very risky because you don't have mid game vehicles.