Lower amounts and worse absorbtion of some nutrients such as zinc, iron, calcium, carnitine, choline. Zero vitamin B12, zero long chain PUFA (only <10% of ALA is coverted to DHA and EPA if I recall correctly). Lower quality (deficiencies in essential amino acids, lower absorbtion) and amount of protein. As a celiac many options for diversification (to better cover e.g. EAAs) are out of question. Same goes for FODMAP. This is why I keep *some* animal products in my diet. Plant foods, including seed oils, have plenty of benefits, but these were arguments against dropping animal foods completely.

All food production kills animals and even whole ecosystems (think rotational grazing vs. endless fields of monocrops), making the killing animals argument kind of silly and predators are part of the natural cycle (I'm not defending factory farming here! We should do it better than that). Many places aren't suitable for planting crops but are perfect for grazing.