I've looked into this before, actually. Everybody says veganism is unhealthy, but India, for instance, historically practiced it quite a bit, and I've learned some things from research.

>vegans are b12 deficient
Vitamin B12 is produced by the microbes inhabiting the soil plants are grown in. One can get sufficient b12 simply by not washing vegetables as well as modern industrial farms do. Furthermore, most produce in western countries is irradiated to kill off any microbes on the produce. This has a secondary effect of removing any probiotics on the vegetables which produce B12.

>vegans are deficient in other vitamins and nutrients
This is true in modern industrialized societies. This isn't because they're vegans though, it's because the nutrient levels in all produce has rapidly declined over the past 100 years. One cannot simply sustain themselves on the shit produce supermarkets pump out, but in other places, like India, with different agricultural practices there's sufficient nutrient levels in the produce to survive.

So yeah, basically industrial society has made it impossible to be vegetarian/vegan, but that doesn't make it unhealthy, and it's probably healthier overall if agriculture weren't the way it is.