Well, iron is hard to get from plants when plants only contain non-heme iron, which isn’t very usable to the human body, and that’s not even taking into account the fact that plant foods are often loaded with chemicals, produced by the plant as a way to discourage predation, that bind with minerals like iron to keep a predator’s body from making use of said nutrient.
Vegan diets are also lacking in vitakin B12, unless one consumes certain kinds of algae.
It also tends to be high in carbs and can be rather low in fat, which is not very healthy. The “healthier” vegan fats (that is, healthier than industrial seed oils) also tend to be high in omega 6 fats and in omega 3 fats, and too much omega fat 6 is no good. Plus, even a vegan diet that is rich in plant omega 3 fats is, in effect, lacking in omega 3 fats because plant omega 3 fats are in a form that is not usable to the human body and are not efficiently converted into the form of omega 3 that is usable to the human body.
That’s just a few of the deficiencies that one’ll run into on a vegan diet without supplementation, and if a diet requires supplementation in order to get adequate amounts of any nutrient then clearly it’s a bad diet. “Nutrient Defiency Man” would be a more fitting nickname.