If a decent percentage do poorly (15-25%), then it is enough to condemn the practice overall - at least in its current form. There aren't nearly enough controls to eliminate this sort of thing, and I think it is probably more expensive to do so. I also think that - apart from test scores - these kids are less likely to be forced to deal with folks different from themselves and a bit less likely to be introduced to concepts their parents disagree with.
Some of the failures can honestly be dealt with if there were proper coordination and oversight: Use an accredited program (for example). Require regular meetings with other students: Say, perhaps, PE at the school a couple times a week at an elementary level. Require parents to pass the tests for the coursework they are planning on teaching the next year or two. Include more variety on standardized tests: If you want to teach sex education or how to follow a recipe, include these sorts of questions on tests.
Until some sort of system is in place to catch the ones that don't really get taught anything other than propaganda, yes, I condemn the entire thing.