There is a very common, though also very silly, picture of Kant according to which as empirical beings we are not free at all, and we are free only as noumenal jellyfish floating about in an intelligible sea above the heavens, outside any context in which our supposedly "free" choices could have any conceivable human meaning or significance. Part of the problem here is that Kant faces up honestly to the fact that how freedom is possible is a deep philosophical problem to which there is no solution we can rationally comprehend.

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.

Freedom is a permanent problem for us, both unavoidable and insoluble.

Clearly no working class movement ever came about that was able to do what Marx was hoping for.

No theory about our bodies as mere objects of observation and calculation (as distinct from partners in communicative interaction, assumed to be free) can comprehend human nature.

Capitalism has not proven to be a transitional form, a gateway to a higher human future.

It was an important part of Mendelssohn's philosophical and religious view that the traditional rationalist proofs for God's existence should be sound an convincing. Kant thought they were not. So Kant's critique was world-shaking for Mendelssohn.