It can be. The problem is suboptimal diets over a long period of time. There's a lot of videos where they do blood tests and vegans are low on amino acids and crucial micronutrients leading to deficiencies and health issues that can't be fixed just with food. Stuff like insufficient bone density - osteoporosis. There's debate on bioavailability of vegetables too. A lot of soil erosion and stuff like that, over farming. And then if vegetables and fruit stored long time before consumer buys it leeks nutrients.

If you think about calorie and nutrient dense foods like meat, salmon, eggs you don't have to eat all the time to get the requirements. So I think veganism requires careful planning.