The Nudity Detection works best on natural photos. That is what it was designed for. Nudity in drawings and paintings may be detected, though the accuracy is expected to be lower.
We do have an Image Type Detection API, that can help you detect if an image is not natural (i.e. is a drawing, cartoon or painting). You could then use that information to adapt your moderation criteria.
We're always looking for advice to help improve our documentation!