Only the most optimistic Marxist can still believe that an entirely controlled economy can work efficiently. Lack of inflation and full employment can be mandated, but a flow of high-quality goods to the public has never been generated by a Communist economy which can match any ordinary Capitalist one. The reasons are not obscure.
1) Nobody knows enough. Large modern economies are too complex for governments to be able to assemble all the information to do rational and effective planning. Stock markets and corporate elites may make irrational and even disastrous decisions, but ultimately their mistakes tend to be self-correcting. These difficulties are exacerbated by the fact that in a planned economy it is not in the interest of most individuals to generate and convey accurate data. The worker lies about how much work has been done, the manager about how many goods are being shipped, and the economist about how successful the latest plan has been. The official statistics quickly become a fiction from which it is impossible to generate any rational plan.
2) People are not motivated to work as hard for the common good in large enterprizes as they are for themselves. This may or may not be “human nature;” but no Communist government has ever been able to inspire its people to work really efficiently except for brief periods, usually through terror. Both the USSR and China had to rescue their disastrous agricultural policies by allowing farmers to develop private plots for their own profit, which typically produced far more than the properly socialist communal farms. Exceptions may be found on small Israeli kibbutzes and other settings where people know each other well, but in large modern states it seems impossible to generate the enthusiasm to work hard except in wartime or similar crises. Toward the end, the Soviet economy was notoriously rife with absenteeism, employee theft, and idleness, expressed in the popular joke “they pretend to pay us and we pretend to work.”
Socialists commonly try to inspire workers with various slogans and visions of the glorious future (say, during the Cuban sugar harvest in the 60s), and these can actually work for a time; but they seem impossible to sustain in the long run. After all, people do not embrace socialism because they want to work harder: they have capitalists to make them do that.