If you can positively ID it and the location isn't contaminated in some way, why shouldn't you harvest and use such weeds/herbs? What can be better for you than things from your own soil that you have harvested with your own hands?

FataMorgana

The warning was likely in regards to poisonous pesticides and antifungals being used on plants for sale. Many "ornamental" plants sold are treated with chemicals not safe for human consumption even if the herb in question is a food or medicine with 8,000 years of human use, lol. It lessens the cost of producing plants if poison can be used.
Thats why people are told not even to eat 'ornamental' chili peppers and 'ornamental' kale.

Go ahead! Eat your dandelions! They are much better first thing in the spring and definitely before they bloom. Once that happens they are unbelievably bitter. Some people cover them with a board so that they are blanched. Of course that will probably diminish the vitamins.

If you are going to eat them,any weed/herb, then you may want to put them in a pot of organic compost. You will get bigger, better & more plant parts that way.Dandelions are tougher the summer lettuce.

Wild foods tend to have greater nutritional and medicinal; value than cultivated plants. As long as they are harvested from a non-contaminated area, go for it! I wild-harvest most of my food in the summer and wouldn't dream of trying to cultivate the wild. Just imagine the difference between an animal in the wild and an animal in a cage...this is the difference in vitality you get between cultivated and wild plants.

I strongly disagree with the suggestion to pot up what you intend to eat. What makes Dandelions so nutritious is their tap roots that mine deep into the soil to draw up minerals. Once you pot up the plant, it has only what small amounts are in the potting mix, plus the stress that comes with pot culture. It seems like a waste of time and effort to pot up what is already thriving on its own.

The only real problem with your herbs is what might be on your soil. Check to see if your area has radioactive or chemical problems from industry or the Government. If no, the harvest away.