*First of all, does it need 18 episodes?* Not really. They could have told the story in waaay less episodes than that.

*Police vs shaman? was the synergy successful??* Again not really. It was already hard to find a midpoint in the harmony between police and shaman to catch criminals and the shaman here is really just a profiler and a hacker.

*Is comedy successful?* tbh Seo In Guk did an amazing job of trying to bring out the humour within his characters and maybe the shaman team did a good job but the police team failed miserably and so when taking the entire story the comedy fell short.

*Characters?* again Seo In Guk did the best out of his characters, but the female lead failed miserably. or maybe her character was really annoying in the first place. Han Jae Hee is a self-opinionated woman, who just believes what she wants to believe. And she is caught up in her own guilt for her brother's death, which also was a result of her own bullheaded personality. And she always screws up Nam Han Joon undercover projects.
The first few episodes are like Nam Han Joon goes undercover --> Han Jae Hee discovers him -->Han Jae Hee screams Nam Han Joon name as loudly as she can --> The bad guys discover that something is fishy --> Han Jae Hee ends up revealing herself as an undercover cop while also exposing Nam Han Joon.
This was soo annoying tbh.

*All in all, was the story successful??* Ummm.... nop. This is sad for me to say really, cuz I'm a huge fan of Seo In Guk and I just feel sad that he choose a project like this. I know he did his best, but the overall story was a failure.