I am sure there are many pitfalls to avoid while having multiple works in progress but I ran into a specific one several months ago which caused me a great deal of extra work and weakened each story.
I prefer having several projects in mid-stream so I can keep working when nothing comes to mind on my main project. That probably points to other dangers related to maximizing creative time and energy but there is one very concrete issue I discovered.
When I looked across my portfolio of projects it became clear to me I was writing the same story in five different works. When I looked carefully at the main characters and what each work was about I saw the same story. Three novels and two short stories about young aristocrats who rebelled against their station in life. I had not noticed this before because I was so busy jumping from project to project, several of which were not similar.
Once I discovered this I reworked two short stories into one, completely changed one novel, took elements of one of the short stories and added them to another novel, and changed the main character in the third novel, which in turn, changed what that novel was about. There may still be similarities between the works but now they each have unique central problems and different, stronger main characters. So while I was writing and keeping my energy going, there was a great deal of re-work that I would not have needed to do if I had been a little more focused.
I still have many projects but I am now careful when starting a new one to look closely and make sure it can stand alone, to see if it is not part of something I am already working on. I also try not to allow myself to be distracted from my main work quite as often. You simply complete more things more quickly when focused on a single work.
Kevin J. Fellows 