A vegan diet needs too much careful planning for most people to get right
It needs some planning, mostly less than claimed. One supplement is essential, B12, and the rest is handled by ordinary fortified foods and a varied diet. Moving from a typical Western diet to a Mediterranean one also takes learning, and being harder at first is a cost to weigh, not a reason the case fails.
Diets only help people who actually follow them. A vegan diet asks you to manage B12 and pay attention to calcium, vitamin D, iodine, omega-3 and protein. Many patients already struggle to take one daily pill. A Mediterranean diet gets most of the same benefits from whole grains, legumes and vegetables, with far less to get wrong.
The list of requirements is shorter than it sounds. The only nutrient a vegan diet cannot supply without help is B12, and one tablet or a few fortified foods covers it. Vitamin D is a problem for many people whatever they eat, and in several countries it is advised for everyone in winter, so it is not a vegan cost. Iodine comes from iodised salt or a fortified plant milk. Calcium comes from fortified plant milks and yoghurts, calcium-set tofu, greens and beans. Long-chain omega-3 from algae is a sensible option, not a requirement on the level of B12.
The bone concern has some basis. Large cohort studies have found somewhat higher fracture rates in vegans, linked to lower calcium and protein intake and lower body weight. That is a reason to eat enough of both, not a sign the diet cannot work. Nor do you need to count macronutrients or combine proteins at each meal. That advice is decades out of date. Eating enough calories from varied plant foods, with pulses most days, covers protein for nearly everyone.
The point about adherence is fair. A plan people will not follow does them no good. But it applies to any change from what someone already eats. A Mediterranean diet is not the default for most patients either. It also means learning new meals and dropping old habits. The difference between that and a vegan diet built on the same grains, legumes, vegetables and nuts is small. It comes down mostly to one tablet and a fortified milk.
Finally, the health comparison is only part of the question. If a Mediterranean diet were slightly easier to follow, that would make it more convenient. It would not change what happens to the animals and fish it includes. Extra effort is a real cost, and worth taking seriously. It is not the same as a reason.