I bet against the EM drive, because that way I win either way. If it's a bust, I won the bet. If it's real, I won new physics!
On that topic, at least a couple of times the rogues theorized about what the emerging force - or whatever - would "push against".
I think that reflect a common misunderstanding. A rocket, or anything emitting particles or energy in one direction to move in another, needs nothing to "push against". It would work even in a vacuum.
Or so I understand from high school physics a very long time ago.
In a reaction engine (like a rocket engine) the gas in the engine pushes against the engine, and then, since it is exhausted out the back, it never pushes the other way. Effectively, the engine pushes against the gas before it is expelled. Or to be more precise, the gas and the engine push against each other.
In a closed cavity, like your microwave oven, there is no thrust because the microwave photons bounce around, pushing equally on all sides of the cavity.
The reactionless engine cannot work because whatever is inside it will push equally in all directions. Claims that it pushes against quantum virtual particles are just gobbledegook, capitalizing on the fact that "quantum virtual particles" sounds really cool.
Anyway, the rogues were not wrong to ask what the drive is supposed to push against because you have to push against something, even if it's only your own exhaust gasses. Which is why any reaction engine needs a propellant, and why a reactionless drive is analogous to a perpetual motion machine.