The Russian revolution did not occur until a generation after Marx's death. He was not involved with it, or with what came after it. His works do not describe post-capitalist society, and a fortiori they do not recommend any part of what the Soviet Union did.

As with many metaphysical and religious questions, Kant thinks they lie beyond our power to answer them. If you can't stand the frustration involved in accepting this, and insist on finding some more stable position which affords you peace of mind and intellectual self-complacency, then you will find Kant's position "problematic" in the sense that you can't bring yourself to accept it. You may try to kid yourself into accepting either some naturalistic deflationary answer to the problem or some dishonest supernaturalist answer.

What are we to think of the shortsightedness of the great mass of people who are content to do nothing about it, and even worse, the greed or venality of the rich and powerful who deliberately bar the way to human survival?

Kant can provide, and has provided, a good model for philosophers to think about the relation of metaphysics to science and scientific methodology.

That Hegel's theory is derivative from Fichte's does not prevent it from being strikingly original and of independent value.

Kant considers belief in God and immortality to be items of "faith" because he relates faith to the pursuit of ends - in this case, the highest good.

The problem is that many who reject Marx do not read him, or read him only by bringing prejudices to their reading that prevent them from understanding him.