The amount of work, research, resources and time that had to be “used” and combined together so this masterpiece could have come to an end it truly remarkable. It’s worth noting that the author worked on the book during the Great War, when his country wasn’t even on the map.
I think the book can be considered a “compendium” of Greek mythology, although I can imagine that the author didn’t mention everything he found regarding to this topic. But I also think that a good compendium is not about the amount of pages it consists but about the quality of information it contains - and I think in this particular book the numbers speak for themselves. Even though it was published in the twenties of XX century, it is still being published and used as the best polish edition of the Greek myths. I wouldn’t say that every book that sells well is worth the recognition it gets, but in this scenario it is the case.
About the Roman myths, I think that historical and religious context is well written, it gives the reader insightful and thoughtful glimpse into the “religious revolutions” that took place in the early years of the Roman Empire. I am not a fan of the Roman myths, mainly because they are a paraphrase of the Greek ones - and a copy is often worse than the original. It’s not authors fault though.
I think the five star ranking is quite unfair, because you can’t simplify such work into a 5 star ranking. In books like that (classics) its often not about the contents, but also about the form and delicacy of presenting such important and antique stories in order to make them last for next generations.