I would certainly agree that there has to be balance in the feed and that the quality of the nutrient source is also very important. Just recently I was diagnosed with iron-deficiency anemia and now need to take iron supplements for the remainder of my pregnancy as well as really focus on increasing the naturally iron-rich foods in my diet. Also important is to consume acidic sources of Vitamin C which increases the body's uptake of iron and limit (to some extent) calcium which hinders iron uptake. I was very surprised to learn that the iron in iron-fortified foods is actually not digestible. There is literally no point in eating those foods to increase your iron supply - it passes right through your system.
As to balance, too much of anything isn't good. A chicken's kidneys will process the extra nutrients as waste. An overage of any nutrients will put extra stress on the kidneys which can result in problems like kidney stones, gout, or even a total failure of the kidneys. Chronic or acute dehydration can also bring about the same issues. I have one hen currently with articular gout and she's the only bird I've had with it in 13ish years of chicken keeping. In her case I suspect a genetic predisposition rather than management. I've had a few cases of bumblefoot, but they occurred soon after we cut down a massive amount of blackberry canes in their primary pasture. The occurrences of bumblefoot have disappeared now that the canes have decayed.
I don't know if this is accurate or not, but have always heard if you cook in cast iron, especially your more acid longer cooked things (like spaghetti sauce, chilli, soup, etc..) it will increase your dietary iron. I've never had a problem so never seriously looked into it, and I always cook in cast iron anyway.