The reason vegan/vegetarian is hard to take seriously is that they *require* supplemenation of omega-3s and B vitamins especially B12 to even do remotely well on the diet.
Any other diet (with meat) you don't have to supplement with anything.
Even though we recommend supplementations like fish oil, vitamin D, whatever else all these things can easily be eliminated by getting out in the sun more or eating more high quality foods like grass fed meat with the fat.
However, something the REQUIRES supplementation (e.g. that which has only cropped up post 1950s) which vege/veganism has isn't exactly comforting to relative health given our past evolutionary history. Especially since meat literally made us human.
I don't buy any argument except maybe religion for not eating meat. And then I don't agree with religious reasons either.