One thing I've noticed in lots of diet advocacy literature is that you'll find statements like "this nutrient is available in that food" without ever specifying the amount or how much you have to eat to make up for the lack of meat/gluten/whatever the diet is trying to keep you from eating.

Quite often people will then build up meals in that diet thinking that they'll have all of the various nutrients they need, but it turns out it doesn't have sufficient concentrations of certain important ones. And you would have to eat multiple kilograms of the replacement food to get enough of the nutrient to be useful.

The amount of nutrition and food science somebody has to know to live healthily on an alternative diet can sometimes be staggering.