The Soviet Planned Economy did suffer a great deal in part because of its inefficient agriculture policy. It pushed an emphasis on Heavy Industry and quite a few of its agricultural "projects" (such as the draining of the Aral Sea) caused multiple problems environmentally and economically.

It really depends on what you mean by "efficiency" however; do you mean by how much it produced? The quality of the products? Speed in which they were made? It's ability to meet all the demands of their people?

Admittedly, I'm biased, I support pretty liberal/libertarian economic policies, but at times you often see centrally planned economies make some truly bizarre economic decisions. For example Mao had an odd interest in steel production as the backbone of an industrialized economy, to the point that a great deal of the mass starvations that took place in the Great Leap Forward could be pinned to him emphasizing steel production over making enough food. You had pseudo-science pushed by folks like Trofim Lyensko where seeds were packed closely together under the assumption that seeds of the same class wouldn't compete with each other. You also had frequent shortages of seemingly cheap and innocuous things like Bananas in East Germany.

The Free Market is, in my opinion, one of the most efficient ways to use resources and produce goods/services. However it could be argued that one of the biggest flaws of Central Planning was that the environment it was in was heavily biased by politics and an attempt to "compete" with the West to the point that you had tangled mess that never really met its full potential.

In a centrally planed economy, you could have politicians decide that they want to "Beat" the Western World in Iron Production, so suddenly resources are turned towards producing the most iron at the cost of other industries atrophying. You had Economic Planners attempt to "experiment" with new crop techniques or huge projects in some gamble to look good politically, only for it to end up a massive environmental disaster.

I feel it's also important to mention that many other modern Communists and Socialists and the like don't believe that a soviet-style, top-down, state-planned economy is the only way to achieve "Communism" or exactly what a Communist economy would look like, but as it stands it's the only example we have.

EDIT: I feel I should also mention that the overall inefficiency of the Planned Economy doesn't mean every good produced in the Soviet Union was a deathtrap held together by duct tape and sheer force of will; Vyacheslav Molotov claimed FDR was a big fan of Moscow's Wine, and the T-34 tank was regarded as one of the finer tanks of World War II. However, overall the Soviet Planned Economy was a mostly inefficient mess dominated by political biases and a heavy emphasis on heavy industry.