I honestly didn't remember it being as bad it is.
Well, ok, calling it a decomposing corpse might be a tad unfair, as this novel is mostly mind-numblingly bland and boring rather than outright bad, and even so it is still miles ahead of Wind of War in terms of quality.
It deals with the struggles of a disgraced Unicorn Battle Maiden
I don't know the reason why unlike the other books in the series The Unicorn one doesn't focus on it's respective Thunder, altough I suppose The Scorpion doesn't either (Kachiko still plays a large part unlike Kamoko though). It is possible that there was no original plan to make the Thunders the main characters of the series and it was something that emerged organically from the writers.
Either way using a obscure character is not necessarily bad, the problem is that Lassieur doesn't really give us reasons to care about Tetsuko, and the entire book is a bit of an exercise in futility.
I suspect, looking at the publication dates, that, at least, the first three books were commissioned at the same time and that there was little or no communication between the authors, and that because of that Laussieur chose not to commit herself to develop a plotline that might be invalidated by later releases. If that was the case she did herself a great disservice, as it did left us, if I remember correctly, with a plot that was invlidated by later releases and some pretty important cliffhangers like Ide Tadaji, that got no resolution. Might as well have taken the risk and try to do a memorable book.
Then again this book might have been her attempt at doing that...
Overall I would say this book is mostly of interest to a completist.
It's not horrible, despite a few weird pieces of characterization like the peasants in former Scorpion lands, the Tadaji storyline, or the Otaku being called Utaku at this point in the timeline which is an obvious result of WoTC guidelines. It's just entirely forgetable. Maybe the writer was just trying to establish how the Unicorn are distinct from the other clans but the author ended up with something that seems entirely diconnected from the rest of setting.