I know not many will see this review, but I believe it's important to point something out.
When writing a story with supernatural elements, you have to represent those elements in an accurate and realistic way so that they don't become inaccuracies in terms of real instances that would exist in this reality.

For example, Angels are supernatural beings that are supposed to obey and follow God's will no matter what. Biblically speaking, that's the correct view of how angels behave. Demons on the other hand rebelled against God and do everything that is against His will.
I could go on lecturing about the reality of God, His will, angels and demons, but you get the point.
The drama itself has many flaws, not just the misrepresentation I already mentioned.
The characters are filled with hypocrisy and don't make logical sense at times. For example, the main lead always has a grudge against the "deity" because she believes he was unfair to her. However, before that, she did not even believe in his existence even though she has witnessed supernatural events first-hand!
Kim Dan, Hu and Gil opposed God's views and always seemed to question Him and even trying to rebel against him at times, and despite the clear warnings they have received, they kept rebelling.....which is completely INNACURATE to how actual angels behave! No angels can become human or vice-versa, thus the reason why the drama falls short in story-telling. Also if the above characters were truly angels, they would have a direct communication with God Himself, but God was portrayed as a silent sadist that had no connection with his angels.

Evil characters were too obvious. The acting was passively boring. Romance was artificial.
And most importantly...the camera work. Why the heck do Korean dramas nowadays incorporate the SHAKY CAMERA cliche in every single instance of the series. To make it look dramatic? It wasn't! To make it look intense? You may have fooled some. I believe that all the positive 10/10 reviews are coming from those that only watched the drama for the visuals of the main lead Kim Dan and Yeo Seo and their poor romance.
It was a drama that could go in great directions but it took one of the worst ones. To me this drama felt more like an experiment for the visual effects artists than for the actual story elements and their reality.