First the positive things:
Very common thriller trope but unexpected plot twists. Also, the acting is amazing, the stories are so well explained, and you can empathize with the characters. I even festival goosebumps with Heejoon's and Wooshik's acting because they have a lot of versatility.

Now the negative things:
I disliked the first +18 just because it didn't have anything to do with the plot and was just unnecessary. I didn't like the way it was so slow at the beginning, but luckily it changed. The last thing I really didn't like was that I think we needed more explanation of Tang's story and his superpower because it came out of nowhere without an explanation.

Overall, I give it a 10 because I really liked and enjoyed this series.