At least our commentary is that before achieving anything that groundbreaking you would obviously have to question the way we interact with physics or interpret them.  Here is the response to why it might be challenging, which is odd because this was sites before by them:

The novel space engine design would seem to produce more energy than is put into it, violating the law of conservation of energy, which (simply put) says that energy cannot be destroyed or created.

“The reason it’s controversial is, it violates Newton’s Third Law,” Brian Koberlein, an astrophysicist who studies general relativity and computational astrophysics, told Space.com.

The whole idea is that this would transcend properties of the 4d world and involve warping space time so that is inherent.  Anyways We hope they are just saying this to tamper things down a bit while they are hard at work researching more.  Here is a full link below to the write up.  Also included is video to reference from NASA themselves on the prototype design.

thanks to space.com for the great write-up

thanks to NASA and The Glenn Research Center for the great image