I agree. In times of severe shortage, major calamities, those following dietary extremes like carnivore or vegan could be placed at a major disadvantage.
It takes some time for the body to adapt to dramatically different diets. Even gut bacteria can be considerably affected by the change. During this time, your immune system could be compromised.
I think another big part of the problem is that while terms like "herbivore" and "carnivore" are pretty descript and specific, "omnivore" is more of a broad catch-all term that people falsely equate to "that means I can eat however much I want of either".
For instance, both Chimps and Wolves are "omnivores" (in that, they possess the digestive tract and enzymes to be able to digest both plant and animal foods)
...but the ratios of each that would keep them in optimal health are very different. (Chimps only get about 5% of their caloric intake from animal foods, for Wolves, that number is 70-80% depending on where they live)
Genetically speaking (and in terms of digestive tract), we're much closer to the former, but a lot of people eat closer to the latter. (Our intestinal composition is such, that we're a
little better suited for fats and proteins than chimps, and they're a
little better suited for plant roughage)
So my gut tells me (pun intended) that for Humans, the sweet spot is probably somewhere in the ballpark of a 20:80 ratio of Plant:Animal
Another confounding factor is that with regards to diet/nutrition, just about every
diet (even the fad ones), are a step up (at least in the short term for metrics like weight/BMI) in comparison to the SAD (Standard American Diet).
When the starting place is so bad, anything is going to feel like an improvement in the first 6-24 months.
If you have 3 people who are 350lbs, low-energy, knee pain, etc... who've been eating nothing but fast food, pizza, and soda for 20 years.
And one goes on a whole foods vegan diet
One goes on a paleo diet
And the other goes on a carnivore diet
Odds are, after 6-12 months, all will have lost a significant amount of weight and "feel better overall"
The analogy I've used in the past to compare is that of smokers.
You take two people who were 2-pack a day (40 cigs) smokers for 25 years....
One quits altogether
The other cuts their cig intake down to 5 cigarettes a day
Both are going to feel a lot better in a year compared to how they felt before.... However, we wouldn't say "oh, look how much better that second guy is doing compared to how he was before, 5 cigs a day must be good for your health"