That's the standard criticism of Up the Walls of the World, and it's justified. For example, it doesn't so much end as trail off, with the characters still ferociously imagining multiple alternative futures for themselves. Tiptree tries to finesse this by having the last viewpoint character conclude, "Let's try it all!" It's not a bad hand wave, because what the characters (and the reader) gradually realize over the course of the novel is that the characters are now facing eternity.
Which is to say that what Up the Walls of the World lacks in structure it makes up for in sheer scale. It's as if Tiptree told herself, "I'm writing a bigger story, so I'm going to expand my speculative scope accordingly." It's pretty literally epic in scale. At times it reminded me of the Star Trek original series episode about the planet destroyer and at others the first Star Trek movie about V'ger. The novel opens with a chapter from the point of view of an absolutely enormous but nebulous space-roving entity that thinks of itself as an evil murderer. I vaguely remember that when I first read the novel I didn't like the use of all-caps to represent the voice of this entity, which eventually becomes know as the Destroyer, because what it does as it roams through space is destroy star systems. I still think the use of all-caps is a clumsy, ugly way to represent vastness, but it certainly didn't bug me as much this time.
So we start big, and then we switch to the POV of a manta ray-like alien living in what seems to be something like the great storm of Jupiter located on an alien planet called Tyree. Tyree is in a star system that's undergoing attack from the Destroyer, and the aliens are desperately looking for a way to survive extinction. Part of what Tiptree has accomplished here is what Gwyneth Jones calls "some the most convincing non-humanoid aliens ... I've ever met." My only caveat is that the characters of the aliens still feel very human to me, and I'm not sure how it could be otherwise without staying out of their minds entirely. But the way they communicate with light and color, and the way they navigate through their environment, have sex, raise kids, perceive the world, all feel very different than any other aliens I've seen depicted in any format, rivaling the Jotoki in Donald Kingsbury's "The Survivor".
On top of these two bits of speculation we are then introduced to a group of humans who are part of a military test of psi powers. The central character in this group is a doctor who suffered a horrible loss in his past and has been self-medicating with opioids ever since. He is skeptical of the experiment, and worse he finds himself overly-sensitive to the pain all of the experimental subjects feel. He falls in love with a black computer scientist, but she is very distant and hard to approach, having suffered a traumatic injury in her own past. The band of experimental subjects is quite various and outstandingly characterized, from the paranoid, to the motherly, to the lesbian couple who are a mix of exuberant and victimized. The only thing all the experimental subjects have in common is pain and fear and lives lived as outcasts, because they are freaks of nature.
Jones says the novel is in a different mode than the short stories -- "a joyous and starry-eyed sf." It's true, but it still has a heavy serving of Tiptree's signature anguish, not least in the genocidal annihilation perpetrated by the self-hating Destroyer, but also more intimately in the fears, injuries, and losses suffered by the Tyreens and the humans. It's one of those stories about endurance of extreme suffering in the cause of a greater vision. That vision does end up being "starry-eyed," but not till the very end. Still, the sheer spectacle of the frantic, star-spanning action and the incredible world-building were enough to keep me happy through all the anguish. The awkward interaction between the aliens and the humans is very smartly portrayed, as is the gradual way they incorporate each other into a new community. This is widescreen baroque SF at its finest, despite the structural problems. As Jones notes, it's also a good example of an ethical solution to the problem of power that doesn't involve domination and exploitation. Echoes of Star Trek in that too?