A close friend of mine was a vegetarian for 7 years, 4 of which she was vegan.
She now eats meat except pork, and the reason she stopped being vegan was because she decided she wanted to compete and couldn't get in competition shape without meat/fish/eggs.
She told me she even notices how it affected her mood and hormones. When she was vegan she, admittedly, said she was a bitch to most people and had a short fuse. Now she's a whole different person.
I'm sure there are some that do well on vegetarian/vegan diets, but just like people who can't handle Keto or a high carb diet, there are some that just can't handle veganism.