It's worth noting that it may well be the only vitamin where deficiency is common in developed countries and where supplementation has plausible benefits. In fact the only thing missing to positively say that supplementation is beneficial is that while deficiency is known for certain to be associated with major issues, and that supplementation has recently been shown to be beneficial in some cases, it's not been demonstrated that supplementing in the general case translates into better outcomes. However, since it's certainly not been demonstrated that it doesn't, it seems quite a bit more plausible than it does than otherwise.

The question is, what are the risks of supplementation? For almost everyone, and at doses that are not completely insane to begin with, the risks seem to be close to zero. But there may be some genetic factors that make a small minority susceptible to overdose.