Both dramas are following a selected amount of detectives chase a suspect over many years. We see how the detectives but also the suspect and witnesses of the crimes change and evolve over time. Both dramas give off a very similar vibe. If you liked one you, might very much enjoy the other one as well.

Both dramas have cases where the killer is playing mind games on his victims. In both dramas characters end up questioning their own morality and have to make decisions that turn them from good to bad.

Both dramas revolve around a culprit that will go to unusual means to open an old case again. Both dramas have a great plot, amazing cinematography and great cast.

Similar story. An old case is reinvestigated.
People will go to unusual means in order to investigate the truth behind an old case. If you enjoyed Trinity of Shadow you will surely enjoy The Post-Truth World.

Both dramas are Taiwanese crime dramas. In both there is a group of individuals that investigates a case that is resembling an old case. The whole time the story questions of what happened in the current case and also how it is linked to the past case. In both dramas there is a culprit that plays with the mind of his victims.

Both drama revolve around a group of people that come together and take revenge on those that wronged them and others.

Both dramas deal around the romance story between a girl and a guy where the guy is not seen as ideal for her. Both dramas are very ascetic and have wonderful background music. Both have a lot of romantic couple moments. Generally they give of the same vibe. The only difference is that there is no age gap between the couples and while in "forbidden flower" the female lead is uprising the male lead, in "one spring night" it is the male lead that is pursuing the female lead

Both dramas deal around a rich girl that meets a poor guy.
In both dramas the girls fall for these guys and pursue them.
In both dramas one of the characters loves to draw.
Both dramas have a similar feel to them and
in both dramas one of the characters has an illness.

Both are crime dramas that follow a group of character over many years. In both dramas there is a protagonist that sacrifices nearly his whole life for justice. In both dramas there is the theme of corruption and the fight against it.

Both dramas are about female leads that get brutally bullied by classmates. Both dramas are quite extreme in terms of portraying that bulling. And both girls take their revenge for what has happened to them in very extreme ways.

Both female leads are very similar in terms of character. They both are the leaders of their squad, they both try to find out what has happened, and are very strong and smart female leads. Also the cases resemble one another quite a lot and have as a theme revenge.

Both dramas have protagonists that start out as good people but go through many things and change for the worse. Both drams revolve around a murder and the revenge of the victims. Both are very well done crime thrillers.