Is Veganism Healthy?

Drew Ramsey says a balanced vegan diet is better than the average American diet, but it's still far from perfect:

Clinical research finds that people on vegan diets commonly suffer from a variety of nutritional deficiencies. One study, for instance, showed that more than half of vegans tested were deficient in vitamin B12, putting them at risk of mental health problems such as fatigue, poor concentration, decreased brain volume with aging and irreversible nerve damage.