One objection is that it gives demonstrably poorer results here than you've experienced.
I prefer an exceptionally clear explanation of a concept, with concrete and pictorial representations perhaps, but without clutter such as description of things as machines when teaching multiplication, etc. Kids don't in my experience need to be babied to that extent.
I wrote a long letter to my son's private school about EM a few years ago. What bothered me most about it was/are a foundation of subtle, misleading mistakes. They turn it into a muddled curriculum.
For example, one way the kids learn "concepts of addition and subtraction" is by using a thing that looks like a calendar. They're supposed to learn that moving their finger to the right and down means "addition" and that moving it to the left and up means "subtraction." This is really just counting, not addition. I suppose the idea might make sense to adults who understand what addition is, but to a kid who has no clue what "addition" means, I can see it being confusing. And DS complained bitterly, saying, "Why can't we just ADD? It's so much EASIER!"
Plus, the numbers are laid out in rows from 1-10 (not 0-9). This layout puts the new set of tens on the line with the old set of tens. This is can lead to misconceptions about where a new set of ten starts. Plus, the calendar in my son's book left out the zero. Why no zero?
EM is also designed so that some of the problems have no right answers. Example: cut out a ruler and measure objects at home. There is no way for the teacher to know if the kids actually got the right measurements.
Don't get me started on asking second graders to cut out pictures of triangles/make collages, write about the presidents who are on coins, or write stories. None of these activities are mathematics (well, the triangles might be pre-math for pre-schoolers).
We carpooled with a girl in 5th grade; EM was even more fuzzy and muddled in her course. I remember something about dropping a flimsy ruler you cut out from the book and dropping/catching it as a way of measuring...something. The force of gravity, maybe? The appropriate experiment would have been to drop a ball from a defined height, anyway. I remember her mom complaining about that.
