The main difficulty with measuring success rates has to do with what you consider success. Some scholars consider retention the main success metric. If success rate means retention rate, then at institutions which offer the same course in both electronic and classroom observe greater completion rates in the classroom versions.
One the other hand, retention rates do not tell you the entire story, as they do not actually specify how much students who were retained learned, if anything. Many diploma mills (including the for-profit colleges such as University of Phoenix) give passing grades to almost all students who pay their fees irrespective of learning outcomes.
As e-learning has become increasingly popular because it is such a cheap way to deliver courses, it is important to define what you mean by successful outcomes in order to assess its effectiveness.