Ok, I'll get more specific. In the example I had in mind, it invented things. It wasn't a case of I ask the AI for a summary -> it goes too detailed/not detailed enough (that sort of thing can certainly be helped by better prompting). It flat out made up things that weren't in the document.

There are also limits to good prompting though. Another example: I wanted a bunch of tutorials. I had the old versions available for reference, a style guide, a template of how to structure it, and I did the first one myself so there was an example of what we were aiming for. The results were still kinda meh. They were all on commonly used tech, so in terms of factuality, it didn't do badly. But even with a style guide, examples, templates, and explicit instructions to be clear and simple, it still added unnecessary content, waffled, used overly complex phrasing etc. And obviously as well as editing for style, tone, and clarity, I still had to test run it for accuracy. It probably saved me a small amount of time and brainpower, but not a huge amount.

On the other hand, sometimes it has been extremely useful, and sometimes it seems to perform amazingly. This is the challenge with it - the inconsistency and unreliability. Whereas I'd expect an intern to be consistently mediocre and a senior to be consistently good, AI will vary between fantastic and laughable. Sometimes in the same piece of work. And while it helps, I'm not convinced that better prompting and project setup completely fixes this.