I don't actually know anything about scribblehub in particular.
You can set continue_on_chapter_error:true to save any chapters that completed with placeholders for failed chapters. Updating the epub will attempt to replace the failed chapters.
You can put a chapter range after the story URL when you download it to limit which chapters are downloaded, example:
https://www.fanfiction.net/s/11111/1/Title[5-8]
Search also
ignore_chapter_url_list and
first_chapter /
last_chapter.