A widespread lie is that the vegan diet is "clinically proven to reverse heart disease". The studies by Ornish and Esselstyn are made to sell their diet, but rely on confounding factors like exercise, medication or previous bypass surgeries (Esselstyn had nearly all of them exercise while pretending it was optional). All of them have tiny sample size, extremely poor design and have never been replicated in much larger clinical trials, which made Ornish suggest that we should discard the scientific method. Both diets included dairy.

Vegan diets are devoid of many nutrients and generally require more supplements than just B12. Some of them (Vitamin K2, EPA/DHA, Vitamin A) can only be obtained because they are converted from other sources, which is inefficient, limited or poor for a large part of the population. EPA+DHA from animal products have an anti-inflammatory effect, but converting it from ALA (plant sourced) does not seem to work the same. Taurine is essential  for many people with special needs, while Creatine supplementation improves memory only in those who don't eat meat.

The US supplement industry is poorly regulated and has a history of spiking their products with drugs. Vitamin B complexes were tainted with anabolic steroids in the past, while algae supplements have been found to contain aldehydes. Supplements and fortified foods can cause poisoning, while natural products generally don't. Even vegan doctors caution and can't agree on what to supplement.

There is an extremely strong link between meat abstention and mental disorders. While it's unknown what causes what, the vegan diet is low in or devoid of several important brain nutrients.