More empirically, there is this study from Faunalytics, which found 29% of ex-vegans and ex-vegetarians in their sample had nutritional issues, and 80% got better within three months of quitting. Their recorded attrition rate was 84%, so if you assume no current vegns have issues that implies a 24% of all current and former vegns develop health issues from the diet (19% if you only include issues meat products cured quickly)