Foul Days really had a lot going for it.

The worldbuilding? On point.
The supporting characters? Interesting.
The folklore? Exceptional. 

It even delves into various aspects of xenophobia, like looking down on certain groups of people but still exploiting their cheap labour and advocating for marginalised groups instead of advocating with said groups.

The only aspect dragging this story down like cement shoes is Kosara, the main character. Kosara's one of the most incompetent protagonists I've ever encountered and, frankly, it's mindblowing how she's lived this long in her city. 

Here's a list of events that made me question Kosara's competence:
•

Kept underestimating the detective even though he saved her from an upir

•

Doesn't detect that the detective "reeks" of magic

•

Doesn't deduce that one of her acquaintances works for the Tsar of Monsters

• Thought a veteran sailor was too "stupid" to transport her safely onboard his ship
• Drugged herself with a truth serum instead of her intended target
• Tried to intimidate the detective with a knife but only succeeded in cutting her finger

In future, I might still try some of the author's other works but only if Kosara isn't the main character.