This question is kinda a slippery slope because it depends on the creator and what they think is considered "breaking the guidelines."
For example, in my novel, I have scenes of gore, mental health, bodies breaking, fights, spines being ripped out, language, and se*ual content tags. I've used all the M tags at least once. If it truly breaks guidelines, staff will contact you, but you'd have to write some pretty horrid stuff for it to get flagged for a novel. Novels have more leeway than comics because it's not visual.
For violence, just use the M toggle to signify that it's an M chapter.
Also heads up, if you schedule an M chapter that has a certain M tag (like se*ual content), your novel will get shadowbanned from the ranking for a day.