Alas, Silver Brumby Whirlwind really makes you realise the series should have ended with Baringa becoming king of the south in Silver Brumby Kingdom.
The tone of the book is maudlin from the start, with the text heavily implying if not outright stating that Golden and Boon Boon have died. At first, this is not completely egregious (the timeline problems become apparent in Son of the Whirlwind), but it makes for a very downer opening, especially with the meeting with Tambo, Baringa and Lightning. Tambo has been away from the Valley for some time; but what has he been doing? Baringa and Lightning do not show any of the characterisation that made them beloved in South and Kingdom.
Benni mysteriously turning into a prophet also makes little sense, though Thowra heading north at least has some logic to it - he has explored a lot of the southern country, so why not go north?
Then we are introduced to Yuri, the daughter of Wurring and Illinga, the two main characters of Moon Filly, and tying the Moon Filly story into the Silver Brumby series. Again, this feels like a step back in storytelling, especially after the lovely romance between Baringa and Dawn, and even Lightning and Goonda. You're constantly forced to remember that Thowra is old now, and Yuri is the equivalent of a teenager. Not helping is that Yuri is flighty, nervous, and gets them into trouble constantly.
The maudlin feel of the story continues with Thowra getting stuck in a cave, two stallions trying to get Yuri (to share? ew!), and the moment when they are nearly killed because Yuri panics. And then, of course, the ending. The story makes it fairly clear that Thowra dies, but later stories have an inexplicable inclusion of magic.
Yuri is a flat character and her introduction really doesn't achieve anything other than to bring Thowra into the orbit of Wurring and Illinga.