There are five main problems,

1. Overpowered enemy
The enemy was almost impossible to defeat. It spent so many episodes, hours and hours of just bullshit all for that last scene of throwing himself off into the ocean. Why didn't the girl know this already?

2. Romance
I didn't watch this for the romance, but if you're into romance, this drama will ruin your day. Actually, there aren't many scenes with them talking, most of it is murder. If you don't like blood and murder, don't watch this.

3. Last enemy
I don't know if it was just me, but the previous enemy Hwang Dae-Du was inside of didn't seem scary like the other two, mainly because he spent a long time being just a side character, like a slave, to the other body he was possessing. He didn't fit the part.

4. Repentance
I know it was meant to happen, but it became annoying after a while, it was no longer exciting or enjoyable.

5. Explaining background
Most of the context of the characters wasn't disclosed or described vaguely, given the basic "My mom/dad passed away when I was little" or "My mom left me when I was little" so they wouldn't have to bother with explaining the parents.

The story is interesting, it had much potential, but the show could've been cut down into like 8-12 episodes. Even though a lot of little details ruined the show, I still liked it for the plot.