Headline says everything. If I were to guess, I'd say that they adapted at least 100 chapters of the novel per chapter of the drama, and even had the balls to add OC to it. It's extremely rushed, and it's no exaggeration to say that at least 95% of the world building got lost.

Read the novel first, then watch the drama. It'll be a better experience for you if you treat the drama as an addition to the novel, instead of a standalone series.

Also, how hard is it to get the license for more than 3 songs? The soundtrack feels so repetitive when you're binge watching the series.