I think if the mom is willing to do the work along the way and learn the Singapore methods, even if they're hard, and be available to teach them to their kids... then it's still the one I'd recommend. I've heard the HIG is excellent for those cases (but I've not seen it myself so I can't say from experience!)
The ones that concern me are the ones that (homeschool or PS) aren't willing to work at it. In PS it might be a teacher who was given the curriculum with insufficient preparation, or who hadn't really wanted to switch from whatever they used before and wasn't willing to make the change, or who, like my own 1st grade teacher, really didn't get math at all (argh!) and figured rote was sufficient and explanations were unnecessary. (double argh!!) I've known plenty of homeschool parents who fall into the "really didn't get math at all" camp too.
But the ones that bug me are the ones that bad-mouth a curriculum at great length, until it comes out that actually what they did was just throw the book at the kid and hope. Gahhhh! Textbooks aren't magic!! I'm certainly not against letting a bright kid take the reins when he's having a great day and just running with it, but when he gets to something confusing then he really needs someone to help him find his way through! And when the grownups (parents or teachers) aren't willing to do that, the curriculum won't save the day.
We're currently using a video program for statistics and even then, with an engaging lecture and clear format, really super-excellent explanations and examples, it makes a huge difference when I'm watching with DS and pausing the video to discuss important points. His needing that little bit of teaching from someone he can ask questions of doesn't mean the curriculum is lacking. And while I think probably a much older kid or an adult could do more with the materials and no teacher, elementary aged kids especially shouldn't be expected to routinely find their own way.
