I don't think Kant's approach to religion is any longer viable in its original form. But that does not mean it is simply wrong or that we cannot learn from it.

Utilitarians are usually empiricists who think they can solve every problem by accumulating enough empirical facts. They do not realize that thinking as well as experience is necessary to know anything or get anything right.

We cannot predict the effects of our actions, especially our collective actions over generations or centuries, to use instrumental reasoning toward these big final ends to tell us what we ought to do.

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.

For the utilitarian, there is a fact of the matter about the good (the general happiness, or whatever conception of the good the utilitarian adopts) and about which actions or moral rules would contribute to maximizing the good. For the rational intuitionist, there are truths about which actions should be done and not done.

As I understand it, Kantian constructivism is partly a position in normative ethics and partly a position in metaethics. In metaethics, it is the position that ethical claims have truth values, but their truth conditions consist not in a set of objective facts to which they correspond, but instead in the outcome of some procedure of deliberation resulting in decisions about what to do.

The problem I see with utilitarianism, or any form of consequentialism, is not that it gets the wrong answers to moral questions. I think just about any moral theory, worked out intelligently, and applied with good judgment, would get just about the same results as any other.