Radical skepticism has a curious tendency to undermine itself: If you can know (or justifiably believe) nothing at all, then you cannot know (or justifiably believe) even that. So it seems that one cannot coherently take oneself to be incapable of forming justified beliefs.
More limited forms of skepticism might hope to avoid this fate. But it can prove difficult to halt the slide once you've started down that route. Consider Sharon Street's epistemic argument against moral realism, which we might reconstruct as follows:
(1) Coherent moral diversity: there is more than one possible internally-coherent normative stance that could survive (procedurally) ideal reflection.
(2) The normative facts are causally inefficacious, and exert no influence on our normative beliefs that might make them tend towards accurately representing the truth of the matter.
(3) Genetic Debunking: If (one learns that) a belief is caused (and maintained) by factors that are insensitive to the truth of the matter, and there are no independent grounds for expecting one's belief to reliably track the truth, then one should give up the belief.
(4) If the normative facts hold independently of our subjective viewpoints, then there are no independent grounds for expecting our normative beliefs to reliably track the truth.
So, (5) Either we lack normative knowledge, or realism is false: the normative facts do not hold independently of our subjective viewpoints.
At first glance, it seems a fairly compelling argument. The problem arises when you notice that the argument could be run again taking the belief in genetic debunking, rather than normative beliefs, as its target. However substantively plausible you may find it, such a sophisticated epistemic principle can certainly be coherently denied. So the topic admits of coherent diversity in opinion. Like pretty much all abstract philosophical claims, the fact of the matter makes no causal difference to the world. So, one's belief in genetic debunking is insensitive to the truth of the matter. There don't seem to be any independent grounds for expecting one's belief about debunking to track the truth. So, by genetic debunking's own lights, one should not believe the principle.
Street's own response is to try to go constructivist "all the way down", holding that the genetic debunking (and constructivism itself, for that matter) is not a mind-independent truth, but one that is true for an agent just insofar as every coherent outgrowth of their current belief set ultimately commits them to it. So she must deny that coherent diversity of opinion on the matter is possible (otherwise these principles need not hold true for the targets of her arguments). But this is not a feasible option. As I show in 'Knowing What Matters', there is a route for the robust realist which is clearly coherent; the only question is how substantively plausible it is, but that question doesn't matter for the purposes of establishing that Street's principles are self-defeating.
The upshot seems to be that, so long as moral realism is coherent at all (as it clearly is), then it is defensible against most epistemic objections. For whatever skeptical epistemic principle you try to invoke against moral realism will (as a similarly abstract, philosophical claim) presumably also apply against itself. (Can you think of a more targeted epistemic objection that isn't self-undermining in this way?)
If we combine this with the observation that metaphysical parsimony arguments don't work against moral realism, the view ends up looking a whole lot more defensible than one might initially have expected...