I think it's difficult to generalize about this sort of thing. Slotting a massive and diverse group of people and their myriad eating habits under these umbrella terms can be tempting, but is ultimately misleading and a generalization. I know some unhealthy vegetarians who just gorge themselves on pasta and junk food all day, and vegans who eat a lot of processed foods, or are way too low on their calories or eat mostly just carrots and rommaine etc. I also know a lot of unhealthy folks on a more typical american diet, or 'paleo' type diet, etc. But on the flipside, I know people on all of these different kinds of diets who seem relatively or sometimes very healthy, due to whatever factors. My one friend is probably addicted to taco bell, but I don't know if he's ever been sick. I don't say this to imply that we should all just eat whatever crap we feel like so long as we are't falling apart, or that there aren't significant impacts on things such as their perception/neural system etc even if we appear 'healthy', but to instead point out that the conclusions we make about this sort of thing based on our own small window perspective of people and their eating habits can be quite tricky. For example, we always focus more on those who don't succeed on a more rare kind of diet than those who do, and this can skew our perspective a lot.
I think it more so comes down to the individual differences, and how they specifically go about doing it. For me nothing else I've tried makes me feel as great as I do now than eating a plant-based uncooked diet of fruit, veggies, greens, nuts, bee stuff etc, but I avoid all of these dietary labels since they hold a lot of baggage and I really can't identify with them anyways (and protein honestly isn't an issue IMO if you really look into this, and in fact too much protein can be a very bad idea). I've been eating this way for 4 or 5 years, eat a ton of food everyday without massive amounts of supplements (although almost everyone could do better by incorporating some, considering how deficient our soils have become, how our produces quality has dropped, and how, IMO, our bodies ability to assimilate nutrients has been heavily compromised in recent times). So far as I have seen I'm not deficient in any key elements, and I feel incredible. Depression has evaporated, social anxiety is getting better all the time, and so on. I've tried incorporating other things such as eggs, dairy, or fish into it, but it just hasn't sat well with my system. I know many who have ate this way carefully for 20 years or longer and are in great health, as well as their children who have been that way for their entire lives. It also seems to have had a massive effect on my psychedelic experiences, and my sensitivity to ayahuasca in particular has skyrocketed- I need about half as much as I used to for the same effects (microdosing probably played a role in this to).
I could get into the details of the other reasons behind why I eat this way but I've already ranted about Tony Wright and other peoples work on the influence of plant biochemistry in our development elsewhere in the forums enough. For me the bottom line is that it just works tremendously well for me, and I know countless people who are more or less in the same boat. As a disclaimer I definitely don't think this is something that's going to work great for everyone right off the bat or at all, or that it's a 'cure-all' as many people like to claim about all kinds of these diets. It's tempting to always blame our diets for any issue we have but I think the overall situation regarding the condition of humans is a bit more complex than that at the moment.
Anyways, considering we are basically primates, and now know a decent amount about the health benefits/detriments of our various foods available, it's somewhat ironic and disturbing that most people I know won't bat an eye at a person or parent who feeds their kids poisonous fast food and junk everyday, but will in the next moment look at you like your clinically insane and on the verge of death if you happen to mention that you eat a plant-based diet. I think that speaks a lot about just how disconnected to reality we've become.
<Ringworm>hehehe, it's all fun and games till someone loses an "I"