Related question: What are the pros and cons of both types of story, from the perspective of writers and readers?
Long-runners have lots of benefits— you can develop characters slowly, showing a maturation and progression. Stories can be grand and epic— you don't have to distill anything. Lots of room for subplots, so lots of things happen in the story. From a writing POV, you don't have to create new characters and a new world every time you write. It'll probably be a steady income.
The downsides: Archive Panic
, as plenty of people just won't want to read a series with 20 novels; lots of them have to be read in chronological order, otherwise things get spoiled; things might get drawn out for too long, or sections might exist as filler to pad out the series; if the first few novels were great, the expectation is to stay great— lots of pressure to please fans. You might get bored of writing the same world, but people always ask for more. Juggling continuity and consistency might be an issue. Juggling plot threads might be an issue. If there is no idea for an ending, plots might meander.
Single novel stories: Not a lot of time to get things done, so it'll have to be concise— but that's actually good
. No long-term commitment— if the story doesn't work, or you don't like the way it turned out, you don't have to keep writing to finish the series. Chance to explore new worlds and characters, develop new and interesting plots, challenge yourself as a writer. You can tell unique, offbeat stories, which is generally harder in a series.
Mid-point: Longer series with definite ends (ala Harry Potter
, Lord of the Rings
)— balance of the two, with both of their problems but also their benefits.
I'm sure there are more. Anyone else want to weigh in?