Marx was part of a wave of thinkers, of which I'd include Freud. These were the early social scientists, who got us to take one framework, and then translate much of the social goings on, in terms of it. It is a credit to Marx, that through descriptions based on class structure, economics and some his theory was convincing enough to lead those applying it to believe it described human life.
But the theory's very inception contains its most critical flaw: Namely, its approach is fundamentally reductionist. Marx, and most of the thinkers that follow him ultimately fall to taking a more explanation and trying to explain a more complicated aspect of human life with it.
Let's give an example. Suppose you have a factory worker, who upon pay day, buys a new truck with the money he's earned. The factory worker gets in his new truck, thinking he's quite happy with his new toy. The Marxist hears the word “happiness” and shakes his head somberly, realizing its his job to pop the workers bubble of delusions.
He proceeds to list the social methods of indoctrination that were used to teach the factory worker to love trucks; like the big monster truck rally he attended last summer. The poor factory worker, the Marxist explains, internalized values which inflated and exaggerated “trucks” into a much more important thing than they really are. These “lies” or “false values”, notes the Marxist, serve the purpose of keeping the poor factory worker in the bonds of slavery. He will keep working for the evil factory, to earn money to buy a truck, which really has no value at all really. Just some fake thing taught to a naive worker.
“But I'm happy,” says the factory worker, “and your theories don't explain it,” And, that of course is the problem with the reductionist approach right there. The Marxist framework, as complicated as is it, isn't nuanced enough to explain the human feeling of happiness. The factory worker feels it very strongly as he drives his new truck home, dreaming of screaming machines, from last summer's monster rally.