Meat is awesome when you need an optimal mix of nutrients — not just empty calories. Of course it's certainly possible to get the right proportions of macronutrients and sufficient micronutrients on a vegan diet but it takes a lot more planning and attention to detail.
not much: meats lacks A. fibers and B. carbohydrate. Some can argue removing B isn't a bad idea, it certainly is quire restrictive. Removing A. have many short and long terms effects that are not very desirable.
Therefore most meat eaters also eat thinks like vegetables, beans, grains etc... which "unbalance" the "right proportions" (if that exist) of meat. It's very hard to achieve near perfect macro and micro nutriments if not with an artificial and perfectly calculated meals taking into account daily physical activity, psychological state, temperature, infections exposure etc... I'm not even sure ISS guys get such a calculation.
> sufficient micronutrients
This is easily done by eating plenty of plants -which is exactly what non meat eaters do- and a pill of B12. One can count but it's not more necessary than if they want a perfectly balanced meat diet, which also have its "problems" when not perfectly balanced.
