A lot of inexperienced vegetarians/vegans end up with nutrient deficiencies because they don't ensure that they are properly getting all essential vitamins or amino acids in their meatless diets. I'm not a dietician, but it's pretty safe to say that these people are not going to be the ones with reduced mortality rates. You cannot simply remove an entire food group from your diet without putting in effort into paying attention to the rest of your diet.
Some studies[1] show that pescetarian diets offer the lowest mortality rates. This diet still requires paying close attention to what you eat, but you are far less likely to have nutrient deficiencies.
This is one of the huge issues I have with diet-mortality studies. During the gluten-free trend a while back, there were many that suggested gluten-free diets provided health benefits. However, for the vast majority of people, gluten isn't really that special as far as proteins go. It turned out those "health benefits" were just the result of people with gluten intolerances actually paying attention to their diet.
Just to reiterate, I am not a dietician. Everything above is just my cynicism for studies that constantly fail to take into account that most people just don't care what they're eating. I would absolutely love to be proven wrong.
While that certainly happens, its not like you have to maintain constant vigilance. Most popular vegetarian diets are nutrionally complete (major exception being vitamin b12 for vegans is something to be very careful about).
Like yes, if you decide to only eat rice and nothing else it will be bad, but its not like you have to meticulously record what you eat every meal. Getting all essential amino acids is pretty easy if you eat like a sane person.
