As a reader of the original Light Novel, I'm biased. I will freely admit this. It is my genuine favorite series and I love it with all my heart. Enough that I often completely forget the rare problematic moments. That being said, I'll be reviewing this as an adaptation.
Warning as this will have spoilers.
Pacing - This season covers 2 volumes of the light novel. Something I think is fantasic. It leaves out little to no events from the novels, fully adapting everything there is. And that's important because everything that happens either exists to set the mood between characters or hint at the plot. However, because it works so hard to fit in every scene, some of them feel rushed. Scenes whose moods would benefit from a moment of silence to let whatever horrific revelation Nephy or Zagan have revealed about their childhoods sink in is lost as it immediately moves onto the next thing. 10/10 for effort but 7/10 for execution. It keeps a relatively high score for not skimping on the content and also maintaining the characters as they are. No one has been altered. I'd say there is one additional scene not in the LN, and that would be the additional scene with Gremory and Kimaris. They are at the auction, but, after they, the aren't supposed to reappear until Vol 3. However, those of us who know these two love them. (We are all secretly Gremory.)
Design - Zagan is described in-book as having a face that could kill on a regular basis on top of being god awful with words. I do not feel this was adequately depicted. They just threw some shadow over half of his face whenever it was supposed to be particularly bad and left it there. In addition, the scene of the demon summoning is integral to the series. It's what hooked me in the first place- the scene that told me that oh so much more was going to be going on here than just two idiots (affectionate) learning to flirt. Thedemon was not nearly as imposing as it should have been and the way it swapped back and forth between it and Zagan "in fear", did not do the scene justice. While the actual design of it was correct, they could have added more to the atmosphere to make it imposing as well as depicted it looming over them to give just an idea of how much more powerful it really was. Because demons are not small (except Samyaza, but we don't talk about him.)
Overall, I rate this highly not because it was a masterpiece, but because it gets the job done that it has to. I hope this series encourages some to consume the original content which, I promise, is even more enjoyable (and has a lot more to binge.)