One of the most frustrating is the way they handled Ohn Jo's father, they were building the story for him to be the big rescuer, and that he will do everything to save his daughter only to die minutes after reuniting with her.
I just have to add that my favorite part was when they were on the rooftop and singing cause that felt sincere.
There are only two characters I didn't hate- Gwi Nam is a scary psycho and the funny delinquent Mi-Jin.