But the reader cannot be sure they ARE understanding what you are trying to convey. Maybe the LLM added something you never thought of, maybe it made form to your various thoughts in a way that isn't actually true to the way you would have communicated them if you did it yourself.

The exercise of writing is an important step of actually understanding your own thoughts properly. Readers are reading to get a sense of your own experience and ideas on the topic, if it turns out you didn't actually write the final thing yourself they'll always feel a sense – in whatever way – of being conned by the author. Intentional or not.

Technical documentation, where what the reader wants to come away with is the clear facts on what something does and how to use it/whatever, is a different matter. But even then editing is key – overlong text, unnecessary information, these will again fight against what the reader is there to get.

With that said, if you do want to do it, a disclaimer is at least upfront and may well show there's no intentional attempt to deceive.