Yes, we Americans overeat beef / Hamburgers, but I don't think anyone eating burgers is at huge risk of malnutrition... as much as a risk of heart problems, diabetes, cholesterol, etc. etc.
It turns out that a standard burger with beef, ketchup, lettuce, onion, and bread covers most of your macro, and even micro-nutrients. It also is way overboard on salt and fats, but... that's not the particular problem here.
One of the bigger problems vegetarians have is finding a proper replacement of meat and all of the easy nutrients meat contains. All amino acids, various vitamins, iron, etc. etc. There's a fair amount of nutrients here.
And even the worst McDonalds burger contains a variety of lettuce / onions / ketchup that covers few nutrients that beef is missing (ex: Ketchup has Vitamin A, Potatoes / Fries has Vitamin C).