It's basically just old people complaining that the way they teach math now is different from they taught it 40 years ago when they were in school (even though the new way is a much better way to make children actually understand what they're learning).
You're correct that most objections are about math, but there are also people who are angry that the reading components are shifting away from dusty old novels with their symbolism and foreshadowing in favor of understanding and being able to analyse the validity of non-fiction texts.
