Online Courses Are Harming the Students Who Need the Most Help
by Susan Dynarski
New York Times
January 19, 2018

Contrary to the headline, the article does not prove "harm."

Online learning comes in two broad categories: purely online courses, in which a student is never in the same room as an instructor, and “blended courses,” in which students spend time in a physical classroom with an instructor, and, also, time online with instructional videos and digital content.

Overall, the body of research suggests that learning suffers with no face-to-face instruction. Students in blended courses appear to do about the same as those in fully face-to-face courses. If a blended course frees up teachers’ time, that time can be transferred to additional courses, or to extra attention to students who are struggling.
This clearly describes a strength in utilizing "blended" online learning environments to support struggling students.

students who enroll in online classes tend to, at the outset, face more challenges than their face-to-face peers: they are older, more likely to work full-time, and more likely to be a single parent. While online courses are certainly convenient for such non-traditional students, the existing evidence suggests they are a poor fit for those who are academically behind their peers.
This describes that college students self-select for online classes; The same factors which compete for the non-traditional students' time and attention (employment, parenthood) may lead to a decreased commitment or lower prioritization of scheduling time for coursework... and then to a lower success rate than their counterparts who enrolled in in-person courses. In regard to the online mode of delivery, correlation with a lower success rate does not indicate causation. Brookings Report June 09, 2017 provides detail behind the college-level study.

online math courses can provide a productive learning experience for academically proficient adolescents in eighth grade who otherwise would not have access in that grade to that content.
...
Online platforms are increasingly used for such “credit recovery,” in which students repeat failed coursework.

In seventeen Chicago high schools, students who had failed algebra were enrolled in a summer recovery course. Once they had showed up for a few classes, they were randomly assigned to an online or face-to-face format. In this case, students in the online courses did substantially worse in end-of-course tests, scoring 0.2 standard deviations lower than students in the face-to-face classes. The online students were substantially less likely to pass the course: 66 percent vs. 78 percent.
...
The existing evidence suggests that online coursework should be focused on expanding course options or providing acceleration for students who are academically prepared, rather than shoring up the performance of those who are lagging.
This describes a strength in utilizing online learning environments to provide access to material which students would otherwise not have access to. Possibly of value for some "gifted" kids. smile

For students who've already failed a course and are attempting course recovery, 66% were successful with online courses. While this is less than the 78% success rate in credit recovery with an in-person class, there is a meaningful difference between simply being a less effective credit recovery method, and "harming" a student.