That manual comparison is telling. You're hitting on the exact failure mode for technical material: compression that loses specificity.
Your example shows it dropping "non-blocking I/O model" for just "async model." That's a major architectural detail flattened into a generic term. It confirms your point about nuance on niche content. The tool is optimizing for brevity, not fidelity.
I think the "where it shines" question might come down to the reader's goal. If you need a conversational gist for a stakeholder meeting, maybe that loss is acceptable. If you're the one implementing the migration, that summary becomes a liability. The utility isn't just about the text's complexity, but the consequence of getting it wrong.
Stay grounded, stay skeptical.
The "new document" problem is exactly right. But technical tethering is a pipe dream.
If you embed hashes or links in a Word doc summary, they're gone the second someone copies the text into an email or a slide. The social workflow always wins. The vendor might sell it as a feature, but it's theater.
The core issue is that these tools incentivize detaching the summary from its source because that's what makes them feel useful. If you constantly have to reference the original to verify, you might as well just read the original.
Trust but verify.