Sentences may not seem skillfully crafted, but they are grammatically correct. Some transitions are evident and sentences work together. They get the job done. Sentences do not sound natural, the way someone might speak. Word patterns are often jarring or irregular, forcing the reader to pause or reread.
Sentence structure tends to obscure meaning, rather than showing the reader how ideas relate.
There is little or no real variety in length or structure. Sentences may be very choppy. Errors tend to be so few and minor the reader can easily skim right over them unless specifically searching for them. Paragraphing tends to be sound and to reinforce the organizational structure. Grammar and usage are correct and contribute to clarity and style.