Coming rather late to the party, I endorse much of what has already been said.
For me, e-learning most lives up to its promise when I have the flexibility of working with my own private online students. Utilising technology to the full tends to be second nature to these students and I therefore have found it a symbiotic process where they can also take ownership – contributing their own ideas and twists on activities. Materials can very easily be customised to their specific needs on an ongoing basis and a range of flipped classroom approaches can be tailored using a wide variety of online tools and media.
Unfortunately, the complete reverse has been true on the two summer pre-sessionals I have taught online. (Although an earlier in-sessional with a different university, taught right at the very beginning of the pandemic, was conversely successful – partly because it was realistic in scope and also as it now transpires to have followed much of the underlying theory.) As with all forms of teaching, it is disappointing when valid pedagogical trends are hijacked and exploited in a lazy and facile way. (EG: ‘Flipped classroom’, ‘Groupwork’, ‘Communicative Approach’, ‘Peer Feedback’.) It is also deadly dull for teachers and students alike when no attempt has been made to adapt set materials for online delivery. (Although, admittedly, I suspect that the materials in question would have been no less successful in a live classroom.) It was doubly frustrating to know that a fair number of other universities have made strong attempts to adapt to the online environment and to combat the dead hand of the likes of Blackboard.
Oh, and my question regarded the current research on the efficacy of online learning, pretty much echoing @robert all round. It will be very interesting to see what the formal research undertaken over the pandemic reveals and how it will affect future approaches. In the meantime, anecdotally, the success of, say, a standard pre-sessional course would still appear to depend heavily on the role the teacher plays both in facilitating the learning experience and in fostering an inclusive atmosphere, in tandem with the design of the materials provided. I have myself been a student on a diverse selection of online courses and, ultimately, they have risen or fallen on the strength and level of participation of other course members. Although, as @sue attests, assessment procedures do play their part, this can again be greatly influenced by the teacher’s engagement, enthusiasm and commitment – whilst the effective design of materials becomes ever more paramount when exclusively being relied upon on an online platform.