So, what can we conclude from all this? When the right genetic (and microbial) elements are in place, vegan diets — supplemented with the requisite vitamin B12 —have a greater chance of meeting a person's nutritional needs. But when issues with vitamin A conversion, gut microbiome makeup, amylase levels or choline requirements enter the picture, the odds of thriving as a vegan start to plummet.
This isn't to say there aren't vegans who really did "do it wrong" (case in point, a diet of potato chips and Pepsi qualifies as vegan), who used their diet to mask an eating disorder or who faced other circumstances that doomed their success from the start.
But science is increasingly supporting the idea that individual variation drives the human response to different diets. Some people are simply better equipped to glean what they need from plant foods — or produce what they need with the fabulous mechanics of the human body.