it is a risk you are taking, and anyway these vegetarians tend to be young, and taking pills because by itself the diet is insufficient. Not in macro-nutrients but choline, chondroitin, carnitine, B12, often K2, B6 if you don’t absorb it well from veg foods, ditto for retinol (in my case, I have poor conversion of beta carotene), Omega 3 fats (we only have a few% conversion of ALA into DHA/EPA, declining with age), zinc, copper, iodine, and often selenium and iron.
In aminoacids, glycin is always lacking (as is lacking in the meat eating population, since they eat only muscle - the same is true for chondroitin which comes from cartilage) but no one notices because everyone is deficient. Taurine is an issue too, although again you will get plenty only with offal.
When I was in my 20s i could survive and even thrive as a vegan, amongst other things because i was motivated to eat beans and vegetables instead of pasta. now I would be a larva and i see it the numerous blood tests that I have been doing online (I do get help from an alternative doctor who is a near vegetarian). I greyed early due to my avoidance of supermarket meat (actually, in 8 years from about 41 to 49), and my hair color (as well as other things) stabilized only after we bought a freezer and then a grass fed steer with other families.
There is also an undercurrent in veg*anism of avoidance of fats which is totally wrong, and also vegetable fats will never be as healthy as animal fats. For one thing they tend to be less saturated and therefore more reactive in the human body. No doubt "plant-based’ web guys like Greger get a lot of funding from Big Pharma.
I note that the two best sites in the Blue Zone, Okinawa and Sardinia, both have an average intake of about 200 grams/day of meat, with lard being the fat of choice (little fish, surprisingly). Sardinians also eat a lot of sheep dairy. I am sure the ideal meat intake could go down if we were to switch to more nutrient dense animal foods (bugs and shellfish), but until then, our guts are quite small and do not have the processing power needed for a nutrient-sufficient vegan diet. The “expensive tissue hypothesis” is basically this. We have big brains and small guts, and nutrient density matters.