It's not so much that common core itself is problematic; it's that in certain districts, kids that don't use it or do the math the "traditional" way get marked down. They arrive at the same answers, but because they didn't follow the same flow as what is being taught, it doesn't matter.
I like common core as a concept, because there are some kids that simply cannot learn how to do "traditional" math without struggling. I think it's great that an alternate method is introduced that is easier for some kids to pick up on.
However, when common core is the only method that's being taught (or more specifically, the only one that's accepted when the grades are based on showing your work as opposed to the actual answer) then it just reverses the problem without solving it. Now the kids that end up finding traditional math easier are struggling and being penalized.