Why do people say Tae Kwon Do is ineffective or weak?

When I hear people on subject of martial arts, they all seem to joke or look down on this particular style. MMA practitioners say "It's just fancy footwork, grab take down" and karate fighters say "Lots of flying kicks and other un-needed attacks" they all have some flaw when it comes to the upper body. I really havent seen or witnessed any tae kwon Do fighting but Im wondering are the flaws that visible?