Veganism isn't healthy
The major dietetic bodies take the opposite view: a well-planned vegan diet is considered adequate at every stage of life, and comes with lower rates of heart disease and type 2 diabetes. The phrase doing the work there is well-planned. B12 is not optional, and a vegan diet without it is genuinely dangerous.
A diet that requires a supplement to avoid irreversible nerve damage, and attention to iron, iodine, vitamin D and omega-3 on top of that, is not obviously the diet humans are best suited to. Plenty of people have tried it, felt worse, and gone back. If it were straightforwardly healthy, it would not come with a reading list.
This is an empirical claim rather than a moral one, which makes it unusually easy to check, and the answer is not close. The dietetic associations of the UK, the United States and Canada all hold that appropriately planned vegan diets are nutritionally adequate and suitable for every stage of life, including pregnancy, infancy and athletic training.
What the evidence actually shows is mixed in a specific and interesting way. The largest UK cohort of vegetarians and vegans found lower rates of ischaemic heart disease and type 2 diabetes, and also higher rates of fracture, particularly of the hip, along with some evidence of higher haemorrhagic stroke. Anyone claiming the picture is uniformly good is overselling it. Anyone claiming it is bad is ignoring the heart disease and diabetes findings, which are the leading causes of death being discussed.
The concession worth making plainly: a badly planned vegan diet is unhealthy, and more so than a badly planned omnivorous one. B12 deficiency causes neurological damage that does not fully reverse, and it is not a hypothetical risk. If someone cuts out animal products and takes nothing, they will eventually be harmed by it. That is a real argument for supplementation and a real argument against doing this carelessly. It is not an argument that the diet cannot be healthy.
And the inference from needs planning to is unhealthy does not hold generally. Wheat flour is fortified with folic acid in many countries, salt is iodised, and milk is fortified with vitamin D in several. Most populations eat a diet propped up by public health measures they never think about. A vegan diet makes one of those supports visible and personal, which feels like a defect and is really just a difference in who is doing the fortifying.
Using a single word in two different senses within one argument, so that a conclusion appears to follow when it does not.
How it applies here: Slides between two different claims: that a vegan diet needs planning and supplementation, which is true, and that it is therefore unhealthy, which does not follow from needing planning.
Drawing a general conclusion from a sample too small or too unrepresentative to support it.
How it applies here: Generalises from particular people who felt unwell on a vegan diet to the diet itself, when individuals respond badly to every diet, including the one being defended.