Even if we were to exclude the fact that different schools train in different manners, which could produce different calibers of martial artists, there are other factors to consider when looking at an art such as Taekwondo.
One of which, being that the art is young and has evolved in many different ways over the past ~80 years. Why bother teaching it to the ROK army, national police, or the U.S. military stationed in South Korea during the Vietnam War if were not effective? During the Korean War, many of the pioneers and their senior black belts served in the military, many of which helped to develop Taekwondo's reputation as "a Killing Art."
For instance Nam Tae Hi, an early member of the Chung Do Kwan, co-founder of the Oh Do Kwan, was well known during the Korean War for his use of Taekwondo in battle. The battle of Yongmun Mountain comes to mind.
I'm not going to sit here and belittle the different "styles" of Taekwondo. It has evolved and fragmented along the way emphasizing Japanese martial arts kata and techniques, Korean martial traditions, Korean nationalism, sport competition, innovating techniques to better serve the sport, etc. Depending on which iteration of Taekwondo you pull lineage, your particular school may emphasize different aspects of the art. Much like the late night NASKA competitions televised on ESPN look dramatically different than a traditional Karate dojo that emphasizes traditional kata and bunkai training over the flash of rhythmic dance and gymnastic feats. I think you can extrapolate which one of the two would be more effective on the street from strictly a technique perspective.
My KJN has mixed feelings about the evolution of TKD. It is one of the most popular martial arts in the world, but with its popularity, the primary focus of the art has shifted to fit the goals of the masses, and that goal is different than it was in the 1950's and 1960's.
I have learned TKD from someone that has effectively used the art in war, self-defense, and sport competition. I know that it has its place in all three situations.