ArniA:
I found the series boring, honestly. When you've seen Ji Chang Wook in Empress Ki, Healer, and K2... you know he's a good actor. But recent series have not been fun to watch.
He's an actor that needs a script and character with high dynamic range in his emotions. Being in a fluffy romcom doesn't suite him I believe, he needs to display extreme emotions I feel like... he does that well.
I think Backstreet Rookie could have been written much better and probably either promoted or aired in a slot that allowed for more mature development of the drama as a whole. If he was American, I could see him starring in an action movie series that has several sequels, and be very popular here. He does have a dynamic range and maybe that is why he takes projects that are not action romances recently.
Overall, I think he is an actor that has skills to match his look and does not belong a list that seems to imply otherwise. It may be an unpopular opinion, but I think Shin Se Kyung is not as bad as the poster thinks and she was really good in Run On.