To tl;dr a tl;dr

• Seeds (read: grains) have the highest concentration of "don't eat me" toxins, because of the role they play in reproduction; phytic acid, for instance, inhibits absorption of several minerals.

• Humans can live off vegetables and some fish (Kitavans) or almost entirely meat (Inuit) and be pretty healthy. However, even animals optimized for eating seeds, much less humans, cannot live off grains exclusively without developing pellegra and beriberi.

• Cereals have exceptionally high energy density, which may lead to overfeeding.

• It's plausible grains interfere with satiety responses via endocrine disruption

• Grains have a bad omega-3 : omega-6 ratio.

• Grains have poor nutrient density.

Distilling Lindeberg's object-level advice: eat lean meat, fish, vegetables (including root vegetables), fruit, and nuts (but not too many). Do not eat grains, dairy, sugar, beans, or processed things. Drink water. I've written about how he comes to this (and my reservations about the lack of extant evidence to support any strong recommendations, including his) previously.