Meat is the easiest way to ensure full nutrition. Especially as the list of known essential fatty acids, vitamins, etc keeps growing with research.
However, there are and have been fully nourished vegan cultures. The key is growing the plants in real soil. (Or Real Soil, as an activist group is named.) Chemical fertilizer is efficient in many ways, but it does not replace carbon in the soil, and it kills the normal soil microbes. These microbes make the essential nutrients typically lacking for a Western vegan. B12 is the best known, but there are many more. Also, a vegan diet has to include fungi, which provides more essential nutrients not found elsewhere outside of meat.
Note that corn fed cows do not get B12, and get sickly without artificial B12 supplements (cyanocobalamin). The synthetic B12 must be detoxified by the liver - this is a problem for humans taking B12 supplements (makes about 1/1000 human supplement takers insane through neurotoxicity). When you eat corn fed cows, their liver has already detoxified the synthetic B12, so there is that.
However, corn fed cows have acid stomachs. Their normal diet of grass creates low acid stomachs, and the natural E-coli are harmless to humans with high acid stomachs. The acid stomach of corn fed cows breed an acid tolerant strain of E-coli which is deadly to humans. So eating corn fed cows is riskier than grass fed.
Furthermore, the fat composition of corn fed cows is unhealthy - reversing the normal Omega 3 to Omega 6 ration. This affects both meat and butter. This is made worse by the CAFO (Concentrated Animal Feeding Operation) environment of corn fed cows. You have the bovine equivalent of a couch potato eating junk food. Not something you want to eat long term.
Bottom line: eat 100% grass fed beef (and pasture raised chicken, etc), and/or fruits/veggies/fungi grown in Real Soil.
We get beef from Polyface farms run by Joel Salatin who has the hands on experience and research in the cows/chickens aspect. He has written a number of books: I recommend "Folks, This Ain't Normal".
In general, for those who aren't farmers, the solution to Big Farm problems is similar to the solution for Big Tech problems. Know Your Farmer. We have talked to Joel Salatin and visit his farm every year or so. Have you talked to your mail admin?
Now I'm hungry