Google states that any copyrighted content is prohibited from participating in web stories, and they'll definitely remove infringing content and links.
Too many words or too much video content
Web story pages are limited to 180 words, and videos are limited to less than 60 seconds. Anything longer, and they might be deleted.
Low quality images and video
Fairly self explanatory - any low quality image or video are prohibited.
Lack of narrative
Web Stories are so called because they have narratives. Anything without a binding theme or narrative structure is automatically deleted.
Web stories must be complete, and anything that requires users to click through a site to see the entire content is prohibited.
Overly commercial web story
Affiliate advertising and display ads are allowed, however if the content is too commercial that it sounds like advertising, then it's definitely not allowed.