Some of it reminds me of the weaker Jack Vance's - an isolated solar system with different wacky cultures - but without his poetry of language. With him, because it is all for fun, you accept the funny cultures and don't worry about how they could have evolved or survived. But Tepper is Terribly Serious about her Themes, so can't have the same licence.
Speaking of Themes, I should mention them. The main one, laid on with a trowel, is how bad patriarchal cultures that denigrate women are. Unfortunately this is a well-worn topic and she has nothing new to say; and the culture she designs2 is so terrible that any chance of subtlety is lost. Here's someone else rather disenchanted with the "philosophy".
The other theme - and here I will be giving some of the book away, don't read this if you still want to read it, though I'm only giving a small bit away - is "how much freedom do you give up for happiness"? Or at least, that's the label I'm putting on a god-like probably-intelligent fungus that spreads underground, and encourage people to spread it. The GLPIF appears benevolent in the book; it only kills bad people, but since these are Really Bad people from the Really Bad Patriarchal Culture, no-one mourns them and the book doesn't really trouble itself with whether killing them ws fine or not. The other thing it does is transmit information, and gently smooth away conflicts in society, so that people don't even notice that the conflicts have gone. This is, unquestionably, messing with people's heads; and yet the book ducks the problem, by having all the main characters agree that yeah, it's all fine. There's also a smaller sub-plot in which another culture, which started with a prophetess1 telling them messing-with-heads-is-bad, who have morphed that to include not cutting their hair. So if some kind of illuminating tension between GLPIF and messing-with-heads was intended well no, it wasn't illuminating.
My own feeling on this would be very much that the book is deceptive, and effectively draws you into the idea that surrendering freedom of mind for happiness is good. In a way, this is what TGTWC does too, in that it's based on Plato. So perhaps I can suggest that Tepper is so unhappy / outraged about the treatment of women in the world that she's going to give up some freedom of mind for happiness. In a book, the GLPIF can be entirely benevolent - or maybe just playing a long game, who knows - but in the real world, the people you're giving up your freedom to are very unlikely to be benevolent.
Notes
1. The prophetess is called Morgori Oestrydingh. Geddit? No, neither did I. Until I just read that the Trilogy is called the Marjorie Westriding series. Geddit?
2. Called the Voorstoders. I didn't recognise it. This footnotes The Voorstoders are obviously meant (in name and culture) to invoke the Afrikaaner Voortrekkers. Well they do, in terms of subjugation of women (at least I imagine the Vootrekkers did; actually I know little about them) but towards the end there's stuff about virgins-in-paradise which seemed to point towards Muslims. Perhaps they're just a confused mish-mash.