I liked this show because it combined detective mysteries with the supernatural. Unfortunately, each episode got more and more involved with the personal lives of the characters, and it was not good.
Detective Finlay (Ben Bishop) wants to commit suicide because the sister of another detective won't stay in a relationship with him. So D. S. Ray Miles (played by Phil Davis), and D. C. Megan (played by Hannah Walters) tell D. C. Emerson (Sam Stockman) to get things right with his sister, so she will go back to D. C. Finlay. Really convoluted and bizarre stuff that is totally off the rails.
We also get the endless scenes of D. I. Chandler (Rupert Penry-Jones) washing his hands, rubbing his temples, and changing clothes at least twice a day because he feels dirty. In one chase down a sewer, Chandler tosses his coat, his vest, his tie, and his shirt, because sewage got on them. Very ridiculous, off the rails silliness.
Even if D. I. Chandler is wealthy and can afford to toss a thousand dollars worth of clothes every day or every week, it is not relevant to the mystery stories, and is distracting. By the time this series ended with this last episode, I was hoping somebody would kill him off the series.
As others have pointed out, Chandler does not communicate with his team, and often belittles the suggestions of other detectives. The series started in a good place, and it ended with a dysfunctional group of characters that are not likable.
The only guy that was usually normal was Detective Miles (Phil Davis), and Chandler was often very disrespectful towards him, while other times he was grateful for his help. I would have enjoyed the show more if Davis had been the top detective, and Chandler character had not been on the show at all.
The other interesting character was Edgar Buchan (Steve Pemberton), who played a history buff and Sherlock Holmes expert during the first two seasons.
By the last two seasons, he was usually in the basement of the police station, washing mold off the walls of his office. Really ridiculous stuff, as A) there are chemicals sold to do that, and it requires removal of the walls where the mold originates; and B) anyone who is breathing mold regularly is going to be very sick and it can lead to terminal lung diseases.
Just having a guy cleaning black mold off the walls of his office in every episode made me wonder what the point was? That the UK has no safety regulations or concerns about its government employees? The the UK is a totally backwards nation? That the character of Buchan had a death wish and wanted to die from black mold poisoning?
Overall, by the end this show was just some kind of bizarre dysfunctional drama which made me feel sorry for the cast. The writers just killed it.