Over the past 5-6 years I've purchased several AI-based writing tools like Rytr, WordHero, Katteb, Voila, Peppertype, Magic Bookifier, and a few others on AppSumo(AS). One distinction Youbooks has from these is its ability to produce 10K+ word outputs. This is a major selling point for me and why I got it.
As a Tier 2 user, I have 200,000 credits/month, and have used about 50,000 credits so far to create two (short) books. In honesty, the book creation process is a big sore point for me. First, the UI is not clear or intutive, and you're left guessing what to enter next. Second, your table of contents (TOC) is suggested input when entering a prompt. Why? I thought that was the job of the AI to create! Okay, so my TOC will give the AI some handholding on which direction I want the content to go, but very confusing regardless. I'm okay with the amount of time the AI takes to generate output, and I like the fact that I can leave the site and be notified when it's done/ready.
I also like the fact that you can select the output file format as Markdown (MD), DOCX, RTF, or EPUB. I wish PDF was there as I have to use Google docs or MS-Word to convert docx to PDF.

Overall, a major leap from the "regular" AI-based writers of years past, but the UI is a work in progress I feel, and it really needs some tweaking! As for generated output, not bad at all!