Interesting subject.
In my own experience, online learning tends to be more efficient than face-to-face, only if the student commits to the process and his/her motivation is intrinsic and needs are properly addressed. For example: I have had a student who took online courses for six months and when she switched tutors her testimony was that the previous tutor was not a "teacher", but a native speaker of the language and did not guide her properly.
Online also provides the "safe" characteristic for some people, that face-to-face does not. That is to say, the student can release his/her fears of being corrected, making the "necessary" mistakes, and is more free to say what he/she does not understand without the "peer pressure" that blocks many face-to-face classes.
On the other hand, for adults, online seems more comfortable as they can arrange times and days that otherwise are a little more complicated.
Of course, online also gives the student the ability to be absent and not having the need to give reasons in person. (that would be a drawback)