The story does not have any solid foundation. Lead characters are not well-developed and you will not root for them.
FL is too stiff. This is probably my first time watching a big k-drama production with a lead this terrible at acting. This is why, you never cast someone noob on a lead role even if they are popular or have connections. She and the writer are the weakest link in this drama. Their lack of awareness of their level of capacity ruined this drama.

Update: So many loopholes and too many unnecessary scenes that did not contribute to the story. Meanwhile, they failed to establish the motivations and relationships of the characters to make the audience feel invested so, apart from Huiju, I wouldn't really care if they killed off all of the characters.