1.) It's named after the Mountain Jirisan, so the expectation is, that the mountain plays a major role in the story. But far away: It's just the place where everything is happening. It could've been named Seoraksan or Wolchulsan and the story could've been exactly the same. No one would notice. That's potential, given away for free.
2.) The story telling is done by jumping between timelines on a regularly basis. (minimum 2 jumps per episode) This is weak, because a stringent plotline can't be built up. The show presents snippets of story parts, glued together. Many times, there is no connection between two adjacent snippets and the viewer always has to guess, if the information gained in the last snippet is relevant for the next couple of minutes or not.
Also, the timeline jumping is most often done by displaying the year the next snippet takes place in. But be aware: Not always! So it's confusing to watch.
3.) The characters do nothing. They are just there and aren't proactive. They wait for the story to unfold. And the viewer waits with them. That's boring! Where is the arc of suspense and the connection between the viewer and the characters!? It's not possible to connect with them on a personal basis. Only (!) in ep 11 the main characters show some degree of emotion.
4.) How many people died in this show? I didn't count, but on average, it feels like it's more than 1 per episode. Why? That's lazy script writing and easy gained airtime, since you can repeat the recovering scene of the dead person multiple times.
If I were the writer and I wanted to let so many people die in my story, I would present the audience a hint on the murderer per death, so they can puzzle around and guess who is the murderer.