They have parallels in that both female leads are caught up in a web of lies; both are ambitious and "cold"; both have personal lives that are falling apart. The genres and delivery is different, so are the characterizations, and while I think Misty is the better drama hands down, MHE reminds me of it.

Both female leads start to doubt people around her, and the moods of the dramas are same. However, the way the FLs react to things are different. In MHE, things are more dramatic, and more directly related to the FL and her past.

Both are intellectually stimulating. Hello Monster is more on the psychological side while WATCHER is heavy on the investigation. The plots are different, but the concept of people being tied together due to a past incident is strong in both; said incident lead to the decisions they made for their careers. The aspect of tension is used too, but executed differently - in Hello Monster, the tension is in characters' decisions and interactions while facing the culprit while in WATCHER it's in finding the culprit.