[Please note that, while there isn't a lot in the way of plot spoilers below, there are spoilers for things that many people really enjoy discovering as they read. If you haven't read at least the first two books but are considering it, I'd give this a pass for now.]
I have to admit I haven't given this question as much thought as I could, mostly because world building isn't something I pay a ton of attention to when I read. Not that it isn't interesting or important! But unless it hits one of my specific interests -- how does this economic system work? does their weather/climate make sense? how is race socially constructed in this society (and what does it mean if it isn't)? -- I don't notice a lot. That being said, I was mostly happy with the world-building in the Steerswoman books, and I'll talk a little bit more about why.
First off, the world that we are introduced to in the first book has some pretty cool features, namely the existence of the Steerswomen themselves, but also the general lack of sexism in the broader society. Women are warriors, soldiers, sailors, farmers, merchants, and even wizards. I feel like a lot of fiction, but this is probably more true of film and television than books, create societies that they claim are nonsexist, but utterly fail to actually show that. Having not just the two main characters be women, but also having many of the people from all walks of life that they encounter also be women was incredibly refreshing to read. As was the general respect given to Rowan and Bel by most characters. They didn't have to constantly prove themselves to people who didn't think women could be strong or smart.
(Sadly, the same cannot be said for representation of LGBTQIA characters or orientations, sadly.)
But when I think about the world-building in this series, I don't think so much about descriptions of the landscapes and social structures as I do the slowly unfolding discovery that the world is not what it seems, and in fact the book is not even the genre we thought it was. Of course this has huge ramifications for the landscapes and social structures and technology and everything else, but it plays out differently than in most books I've read because of the way that the reader is discovering things alongside the characters, as well as being given exposition about the world that the characters already know.
Speaking of exposition, the steerswoman, with her emphasis on seeking and sharing information and thinking things through carefully, is a nicely non-clumsy voice for explaining the world to readers. She's kind of like Giles that way. :)
Anyway, I liked how aspects of the world building that initially seemed incidental, such as the units for distance used by different peoples, turns out to be important clues. When Rowan uses miles and feet, I didn't really think anything of it, but later when Bel uses meters and kilometers, the world they are in suddenly seems connected to the world that we are in in a very different way than I originally imagined. The world at the start of the book seems like a less patriarchal version of the sort of mythic vaguely late feudalist or early capitalist Europe that is standard fare in many Western fantasy novels. By the end of The Outskirters Secret, it seems much more likely that their peoples are the cultural descendents of Europe and the people that Europe has colonized.
The reason I put it that way is because while the names of characters and towns suggests western European origin, the range of skin color and hair color and texture that various characters are described as having, along with a complete absence of discussion about race, hints to me that this is meant to be some sort of "post-racial because everybody is mixed race" (or, I suppose, "everybody is mixed race because we're post racial") world. Which is not something I'm super fond of, but I'm glad at least that many characters are described as dark-skinned. And perhaps I'm being ungenerous in my interpretation.
Also, I know this is not important to very many people, but I have to give Rosemary Kirstein a shout out for some very impressive dynamic meteorology in The Outskirters Secret. Her descriptions of what kind of weather would be caused by a very intense heating of a huge strip of land was pretty on point. I secretly wanted a short story where Rowan establishes the first steerswoman school of meteorology after those events! And actually speaking of weather and world building, it is a bit funny that the Steerswomen are documenting every change in every stream but don't seem to have any weather stations. I would think at the very least they could manage a mercury or alcohol thermometer and a wind vane in each town. And I do remember some generally confusing inconsistencies in what kind of knowledge and technologies Rowan's people had, although I can't think of any examples right now.
At any rate, there are still so many unanswered questions in that series, many of which revolve around the origins of Rowan and Bel's societies and how they came to not only lose a lot of technology but apparently forgot that they ever had it. (Along with, of course, what the hell Slado is up to, but that doesn't fall under world-building in my head.) I have to admit that I had hoped that after four books that I know more about the world and its back story then I do, but I also have faith that it will all be explained in due time. Assuming anyway that Kirstein finishes the series. In the meantime, I am thoroughly enjoying the discussion on James Nicholl's reviews. As well as being reminded that I should re-read these books because I've already forgotten a ton.