For me I would probably say writers block. Although I obviously still have it when I'm writing a regular novel, I feel even more pressed with a webnovel. With a novel or work that I'm not publishing until its completed, if I have writers block I can easily just take some time off to edit previous chapters, do research, or write ahead. However, with an on-going webnovel, having writers block for too long can mean loss of fanbase, drastic change in writing style suddenly (which can be good and bad depending on how it's done), and sloopy, fast-pased writing that doesn't immerse the reader.