* While promising the potential to be a learning tool, the computer is also an addiction. I've observed that it's almost impossible for some kids to manage their time, and to maintain their focus, while doing lessons on the computer. This has been a huge setback for one of my kids. Fortunately, math is still done on paper, so he gets something out of that.
* I've looked at the online lessons. My impression is that the effort of programming the interactive environment for online lessons tends to limit the breadth and depth of those lessons. Math instruction has abandoned proofs. A huge amount of the computerized lessons are busywork.
* There's no limit to the amount of homework that can be pushed on kids.
This is nothing new. Roughly 30 years ago I had an internship at an educational computing center that had almost every kind of computer and educational software title in a demo lab, for teachers to try out. The vast amount of apps amounted to glorified flash cards.
How great it could actually be is lost on the teachers. Instead of working through canned lessons, or surfing for stuff to paste into a report, kids could explore "real" software such as (just listing some of my favorites), Scratch, iPython Notebook, Arduino, etc.