I don't have any experience with Udacity, but I do have an experience with Coursera that caused me, the student, to shy away from their courses.
I completed a course through Coursera from the University of Toronto. It was a good course, and I enjoyed it. Learned a lot from the course. In the final week of the course (it wasn't a free-for-all - I had to register for the course and complete it, with tests every week, during and eight week period set but the U of Toronoto), there was an exam that would make up 50 percent of my total grade. Coursera completely fell over that final week, and I wasn't able to gain access to the test until two days after the course deadline. So there went an otherwise good grade. They wouldn't allow any tests to be taken after the deadline, regardless of technical issues.
I had spent a total of around 40-45 hours with the course, 20 of those hours were video lectures that needed to be watched, the rest was study time. Even though all I would get from the course was a certificate of completion, I felt cheated and like I'd wasted a lot of my time for what was otherwise a good course.
Would I take another course? Maybe, but I know that if I were studying for transferable college credit, I would have been seriously pissed.
I wonder how much of the non-pass rate was due to issues other than actual class material in Udacity's case.