The biggest limitations on using AI as a writing assistant is it is limited to just I think 14000 character buffers of what is in its "memory". So if you want to give it a large document as input, it will forget everything except the last 14000 characters. And if you have a long conversation with it, it won't take very long before it has lost all the context you carefully built up. It forgets what it is working on or what the rules were, and you continually have to remind it of the constraints you wanted to put on the content. I've used it to a limited extent, but it's not nearly as useful as I would like.
It's also not nearly as imaginative as it might first seem when speaking to it. And worse, it's output becomes part of its input so it rapidly overtrains itself and gets stuck in simple patterns.