Seems a lot of reviewers don't get the series at all.
Could there have been more development? Yes. Could they have provided more context? Yes. Does the show make sense and, is in fact, great without them? Yes! The lack of certain context and the mystery around these otherworldly being are intentional and, assuming there's a season 2, will be explained in due time. This isn't a movie or some type of drama where EVERYTHING is explained or narrated for you. It's meant to make you think and it's meant to keep you in suspense and guessing. It basically throws you into a world where the story has already progressed and you try to make sense of it from the perspective of someone who is living in that moment as a bystander. You can see this from the moment the series starts when the cafe gets destroyed by the monsters. And if that's not how you like your stories to go and you need to be spoon-fed all the context, then this isn't the series for you.
Also the underlying themes of the show isn't just about how terrible people are/can be, it's about how fear influences the decisions/beliefs that we have as a collective. It's about how society is only a stone skip away from falling into completely anarchy when the unexplainable/unquantifiable oppresses us. How society would collectively throw common sense out the window to take comfort in an obvious lie and how some would take advantage of that and call themselves just/righteous while doing it (even justify murder of a child).
There are also specific themes that would resonate with a Korean-centric audience. The broken system that is the the underage children protection act. The influence of certain religious leaders in government and the police. The power of influencers in modern day Korea right now and how what they say goes largely unchecked/unmanaged as long as they have a large enough following.
Also why would anyone compare this series to a series made by HBO/CBS/other well-funded, established studios? The studio that made this series is tiny and the entire series was budgeted at 15mil. For context, a SINGLE episode of Game of Thrones was 15mil. The visuals on this, while not great, did accomplish what it set out to do and that was to create grotesque, unexplainable creatures that burned people. You want better CGI? Go watch Game of Thrones or the Mandalorian.