"Controversial" seems to be scientific-journalist-speak for a new theory or hypothesis which is not widely accepted and/or which does not have much experimental evidence to support it.
Encouragingly, the evidence for the EmDrive seems to be rather stronger than for the various attempts at cold fusion: six separate groups have each built their own apparatus and got the same result. Even if the theory is not right, it's looking as if there is a propellant-free propulsion technique ("space drive", if you like) that we can make use of. If we get to understand inertia as well, that opens up a whole load more possibilities.