It must be understood that "animal rights" vegans would benefit their cause more by promoting the health benefits of eating vegan. The ethical argument is not going to be won because "animal rights" activists do not have superior numbers on their side and they are not acknowledging the difference between wild/farmed/pet animals. However, the health benefits of a vegan diet have medical and scientific justification which affects the majority of people. This is a numbers game. Victory is achieved by getting the numbers.

The worst idea I have seen in veganism is the notion of eating "clean". Anyone who has a smattering of understanding of microbiology knows how absurd that idea is. Fruits and vegetables are not "clean" fuels for our body. Hopefully they have plenty of helpful bacteria on them to contribute to our gut population. Refined sugar and refined fats are "clean" fuels and should be avoided because of that fact.

The biggest problem with the idea of eating "clean" is that it seems to lead to delusional aversions. People think their diet has led them to a type of piety where they become more sensitive to "unclean" things. They then can't tolerate certain smells or be around certain things or become nauseated by certain tastes. This might spiral into debilitating neuroses (anorexia, bulimia, hypersensitivities, etc.). Eating "clean" or "pure" or "perfectly" is the first sign of an eating disorder. It is healthier to occasionally and intentionally break the spell of such thinking than to allow it to have a pathological stranglehold.