My evidence is anecdotal: I used floss picks for months, went to the dentist, and he accused me of not flossing. My sister is a hygienist, and she told me that

My evidence is anecdotal: I used floss picks for months, went to the dentist, and he accused me of not flossing. My sister is a hygienist, and she told me that's her experience with patients as well, and that you need clean floss to avoid spreading bacteria into the places you're trying to clean. Optimal form may also be hard to achieve with floss picks - you want to form a 'c' shape with the floss partially wrapping around your tooth. That's impossible to do with a short, taught piece of floss.