"If we listen to science about herbs and all the possible ways of getting poisoned (by things like oxalates), I would have died years ago".
Oh yes. That's interesting in itself!!! For example, the ongoing debate about comfrey tea, or amanita mushroom fucking with your liver.
We do not, for example, eat foods that contain only oxalic acid, food contains so many other components that might offset the effects of, say, oxalic acid.
Yes! It's so complex. I was listening to an audio book that was talking about nutritional dark matter - there's a whole world of nutrients in plants we don't understand, not in isolation, and certainly not how they might work in combination with other plants. Let alone the way plants might, say, work in various ways. For example, one plant might start you up at the same time as slowing you down, or rev ME up but put YOU to sleep. No wonder science finds it hard to unequivocally say ANYTHING about plant medicines.
Oh and let us not even start about our gut microbiomes that differ so much! I mean, as you said, we can eat the same thing and our bodies can extract different things from them and this can influence us differently, and so on. The world of nutrition is actually so complex that I cannot fathom nutritional science and "nutritional experts" preaching their outmoded stories. In South Africa, the heart health association and one of the leading life insurance companies work together. They do a very good thing to get people active and they reward good behavior (which sounds like a good thing but is actually not good, but I digress) but their "nutrition model" is based on the "fat is bad and carbs are good" model. In their one pamphlet, we received they advise you to use up to a cup of sugar!! Just on the idea of gut microbiome, you are feeding the wrong bacteria with that much sugar, killing the "good" ones in the process and there you have a nightmare just waiting to happen.
Sorry, long reply just to state a simple thing: the impact of our diet on our microbiome and how this affects how the same food gets broken down and subsequently absorbed by different people, is a fascinating new world!