I think that if you have healthy birds then you can consider your eggs healthier. I guess if you fed them garbage they might be less nutricious but it is not like cows milk where the intake of food directly effects the actual product. Like when cows go from alfalfa to free range or pellets the milk can look and taste funny until you get used to it. My chickens eat bugs but the eggs don't necessarily taste like bugs. Yes, garlic and onion can have an effect but eggs are not created in the process of digestion of actual food - more from base components via the reproduction line in a chemical way like a human fetus is fed via the mothers actual blood just like any other organ in her body not directly through the digestive system. It is a different process from components that have completed the digestive system and have gone into the blood stream as chemicals more like atoms than food just like oxygen, sugars, and proteins.
Caroline