It has been argued that if we are the product of unguided evolution, we would not expect our moral sense to get the moral facts right. I think there is a lot to those arguments, but let's suppose that they fail, so that there really is a good evolutionary story about how we would get a reliable moral sense.
There is, nonetheless, still a serious problem for the common method of cases as used in analytic moral philosophy. Even when a reliable process is properly functioning, its reliability and proper function only yield the expectation of correct results in normal cases. A process can be reliable and properly functioning and still quite unreliable in edge cases. Consider, for instance, the myriad of illusions that our visual system is prone to even when properly functioning. And yet our visual system is reliable.
This wouldn't matter much if ethical inquiry restricted itself to considering normal cases. But often ethical inquiry proceeds by thinking through hypothetical cases. These cases are carefully crafted to separate one relevant feature from others, and this crafting makes the cases abnormal. For instance, when arguing against utilitarianism, one considers such cases as that of the transplant doctor who is able to murder a patient and use her organs to save three others, and we carefully craft the case to rule out the normal utilitarian arguments against this action: nobody can find out about the murder, the doctor's moral sensibilities are not damaged by this, etc. But we know from how visual illusions work that often a reliable cognitive system concludes by heuristics rather than algorithms designed to function robustly in edge cases as well.
Now one traditional guiding principle in ethical inquiry, at least since Aristotle, has been to put a special weight on the opinions of the virtuous. However, while an agent's being virtuous may guarantee that her moral sense is properly functioning--that there is no malfunction--typical cognitive systems will give wrong answers in edge cases even when properly functioning. The heuristics embodied in the visual system that give rise to visual illusions are a part of the system's proper functioning: they enable the system to use fewer resources and respond faster in the more typical cases.
We now see that there is a serious problem for the method of cases in ethics, even if the moral sense is reliable and properly functioning. Even if we have good reason to think that the moral sense evolved to get moral facts right, we should not expect it to get edge case facts right. In fact, we would expect systematic error in edge cases, even among the truly virtuous. At most, we would expect evolution to impose a safety feature which ensures that failure in edge cases isn't too catastrophic (e.g., so that someone who is presented with a very weird case doesn't conclude that the right solution is to burn down her village).
Yet it may not be possible to do ethics successfully without the method of cases, including far-out cases, especially now that medical science is on the verge of making some of these cases no longer be hypothetical.
I think there are two solutions that let one keep the method of cases. The first is to say that we are not the product of unguided evolution, but that we are designed to have consciences that, when properly functioning (as they are in the truly virtuous), are good guides not just in typical cases but in all the vicissitudes of life, including those arising from future technological progress. This might still place limits on the method of cases, but the limits will be more modest. The second is to say that our moral judgments are at least partly grounded in facts about what our moral judgment would say were it properly functioning--this is a kind of natural law approach. (Of course, if one drops the "properly functioning" qualifier, we get relativism.)