I tried serials on Medium several times over the years and they never did well for me. On Substack, you know your subscribers will get each part. So they are more likely to get read.
Substack requires some experimentation to find what your readers want (it helps to ask them.) Some Substack fiction writers do a weekly serial. Some do short stories, flash, or microficiton. Some a combination of all. Some also add non-fiction (writing tips, commentary, backstory, publishing news, etc.) It is really up to you to make it what you want it to be.
The main thing is to be consistent. If you think you can send out 52 stories or story parts per year, it might be worth a try.
There is a Discord server (sort of a chat room) call Substack Writers Unite that Elle Griffin started. It is very active with writers discussing how to make Substack work. You might want to poke around there if you are interested in Substack. Here is an invitation to join:
