The soviet union was less efficient in that it allowed compradors to take power and end it. It was a less efficient proletarian-dictatorship than the rival bourgeoise-dictatorship. Its principle mistake were structural mistakes allowing the liberals into power.

Yeah having all of their cities and a quarter of their population destroyed will do that.

Under Stalin and Lenin it was extremely efficient, growing and developing the economy at extreme rates unseen in human history up that point until the German invasion of WW2. The destruction of WW2 and the Liberal-revisionist take-over of the post-war government after Stalin's death led to liberalization and stagnation right at the time where they needed to recover the most via command planned economy. Kruschev's social imperialism and revisionism led to the Sino-Soviet split.

There was a failure there, a failure in purging out the social-imperialists and revisionists like Krushchev and his ilk.