All in order:

1) The acting is very good!
The characters portrayed are complex, multi layered, with intense back stories and sometimes unclear path they will go. The cast deals with this wonderfully, portraying the full range of emotions as intensely as needed.

2) Cinematography and soundtrack are also good.
You wouldn't call this a cinematic masterpiece, however the setting, the coloring, the added music add to the eerie, unsettling and often difficult to grasp events and settings.

3) Timeline jumping.
For more than first half of the series we jump between 3, sometimes 4, timelines - what the detective is telling, the interview with him that is taking place, the mass murders interview and different memory sequences.
We also jump between character POVs - the detective, the murder, the teenagers looking for their friend and several more.
All this can be quite confusing and throw one off the plot.
All timelines merge in one near the end and becomes a much easier watch. Apart from some flashbacks revealing character back story.

4) The plot itself.
Cults, brainwashing, hypnosis, gas attacks, mass s**cides and much more. Many of us will recognize elements from actual events and cults inspiring this. The plot is actually very interesting, however the executing with different timelines and the many POVs takes away from it.

Overall - a good background watch that picks up the pace and becomes more clear during the second half. But would I recommend it to everyone? Probably not.