TL;DR - Unique and charming in its own way.
There are a lot of really unique and interesting aspects to this manga. I think the best word I can use to describe it is whimsical. The "mysteries" are only mysteries to the reader - Iwanaga, the main girl, is the "goddess" of the youkai. She's a human, but she was chosen by the youkai to be their goddess. She takes requests from various youkai to fix their problems and solve various mysteries. It's a twist on the classic "service club" trope - except it's not a club, and the folks they're helping aren't other students. And Iwanaga seemingly has all of the answers and explanations.
What makes this really interesting is the way the manga weaves everything together. You have much more mature, sometimes dark themes. There's a big focus on character development, and sometimes it gets a little gory too (also a long story...). As another reviewer mentioned, Iwanaga's boyfriend is actually immortal and has the ability to somewhat manipulate destiny. So a lot of their battles revolve around him dying, altering reality a little, and then Iwanaga using that as a chess piece on her board.
Some of the mysteries and characters have been really interesting and intense. But some of the "answers" to the mysteries have also been kinda far-fetched and a bit of a stretch, hence my docking a few points for story.
Overall, it's a slow burn. You won't find a lot of fast-paced, shounen-esque action. Rather, you'll find a whimsical telling, meandering, of little pieces of information that finally culminate in an answer that may or may not explain everything. Chapters are super long. But once you get into it, it keeps you curious about what happens next.
I feel kind of bad for saying this, but what keeps me reading is actually all of the characters surrounding our protagonist and deuteragonist. Iwanaga and Kuro (the bf) are fine, but it's really the youkai and the subjects of the mysteries that are interesting.