I stopped drinking milkshakes altogether after I once had a really bad one. It was in a seemingly decent stall on the street, with refrigerators and everything. I suddenly strongly disliked its overly sweet taste, I'd rather have a cola. In general, plain water is the best...
Well, never mind.
McDonald's simply can't afford to violate sanitary standards, unlike various shawarma makers, for one simple reason. It's a well-known brand with a reputation for quality and popularity, and its roots go back a long time in America, where everyone sues everyone else. If it were as bad as some people describe, Mr. Ronald would never be able to pay off his debts...