Perhaps it has partly to do with subject matter. My DD has had flipped classrooms a couple times for science; her AP bio teacher this year uses that approach, She loves it. There is a relatively short video lecture most nights (or kids can use the textbook for the assignement, whichever the student prefers) and then the kids are set free the next day. All of their labs are self-designed, so she and her lab group, who have no problems with understanding the presented material, can spend their time applying the lessons without being slowed down by the need for questions and clarifications from other stiudents. (Our school encourages all students to take as many advanced classes as possible, so large variability in student readiness for the classes persists, just like in the non-tracked middle school classes, but that's another issue.) The kids who need help with questions have direct access to the teacher, often one-on-one or in small groups, while,the kids who don't need it aren't held back. I can't speak from the experience of weaker students, but for my kid it's fantastic. If she is organized and speedy, sometimes after working on the lab she is able to complete homework, lab write up stuff, or work on the next day's lesson during some of the class time. This is a huge bonus in her day, which is more often than not filled with unnecessary but mandatory review/repetition and slow pacing, leaving a boatload of homework every night, which will be reviewed in painstaking detail during class time the next day, a real waste of time from her perspective. (Especially, as spaghetti noted, there are lots of kids who just don't bother to do the homework, doubly punishing those who did.)
I'm not sure how math would lend itself to this format, honestly, but for lab science I think it's great. I't does help that the video lessons are well-done, and also that a textbook option exists for those who learn better that way. I know my DD wishes more teachers used this format.
