That EM drive took (and still takes) physicists by surprise, and its design must be terribly frustrating, because it doesn't leave a way back out to some kind of acceptable explanation, outside of a purely QM model. :^) It's an absurd situation, not least because there appears to be no plan to test it in space. A test bed should be orbited, pronto! :^) Gwynne Shotwell of SpaceX stated not long ago that they're doing work on nuclear propulsion, a system that has never shown practical feasibility and was abandoned in the 1970s (it can't get itself off the ground, it only works in space). Seems like EM drives (radio frequency resonant cavity thrusters) would be cheaper and better and easier to recycle after each use, but hey, the mighty Elon has spoken, the smoke has gone up on each side of his face and everything. :^D
Except that the oligarchy still seems to be trying somewhat to squelch such advances from reaching very public venues and uses.
The pace of the space testing effort by the academic physics community though may seem leisurely, but this is likely because academic physicists would want not incremental but definitive results. That requires building one or more fully instrumented and robust EM Drive models that are thoroughly ground tested in order to eliminate all possible technical faults, measurement errors, and alternative explanations for any thrust that might be observed in space.
Ideally, a space flight test also ought to resolve which theoretical physics explanation for EM Drive thrust is true and to provide guidance for how the design might be improved. As it is, there is no consensus as to how the EM Drive might work. The two best explanations in my layman's view are Unruh radiation or that the de Broglie-Bohm pilot wave theory of quantum mechanics is true. If so, the EM Drive would revolutionize modern physics.
In any event, if the EM Drive works, whomever first explains and proves up the physics can expect that their future will include an early morning call from the Nobel Committee in Stockholm.