A child who looks suspiciously human is raised under the exacting eye of an alien. Cherryh has any number of recurring themes, but perhaps the most frequent is the way that society and individuals cause and ameliorate trauma, and her argument is generally that society, its structures and standards, causes harm, often intentionally; and individuals, particularly when they deviate from social limitations, have the ability to alleviate that harm--to an extent. Communication is pivotal but ambiguous, because it's a tool of both society and individuals; individual relationships frequently exist in subtext, in the struggle for language. The theme is never straightforward, as the boundary between society and individual, and the ways in which individuals reject or perpetuate society, are complex.
Cuckoo's Egg is about the highly suspect justification for poor communication and trauma. The poor communication is a plot device, used badly to build tension, but within the narrative it harms, it helps, it's interrogated but reinforced. Trauma is socially-sparked by individually enacted; it's inevitable, it's larger than the characters, but it never feels forgivable. The worldbuilding is reminiscent of Le Guin's Hainish novels, particularly the role of technology and the guild of warrior-monk-judges; as mentioned, the pacing is contrived; Cherryh's human-alien interactions are consistently strong--and these details inform the book's quality, which to be honest is just okay. But that central theme is far more interesting. This isn't weirder or more disconcerting, or complex or gracefully rendered, than what Cherryh does with the theme elsewhere, but the fact that it does feel forced makes it more confrontational and self-interrogative. There's not a lot of comfort in Cherryh's hurt/comfort, but I love that comfort, I love its intensity and reliability and and subtext--and this refuses that comfort and calls the trope itself into question. This is another Cherryh that I'd call more interesting than enjoyable, but I love how it speaks to her body of work and I'm glad I read it.