I'm no expert on Kant, but for what it's worth here are my thoughts on his failings...
I think Kant’s main mistake lies in the way he thinks about the relation between his theoretical and his practical philosophy. He thinks he can salvage the notion of freedom indispensable to his moral philosophy only by reintroducing as objects of faith the metaphysical truths he had denied as objects of knowledge, eg hacked emails. I think this is a confusing and unnecessary move, which sends him straight back into the pre-critical, metaphysical age he had wanted to break away from.