Earlier quoted context omitted.
Basically, it's because as the drive keeps accelerating without losing mass, the velocity is proportional to the energy expended. However, it's kinetic energy increases with the velocity squared. Eventually, there is a cross-over point. You can create infinite energy by placing a bunch of drives on the spokes of a huge flywheel. See this paper for a more detailed answer: https://arxiv.org/abs/1506.00494
My argument still stands you cannot ever reach such a cross-over point at the milli-newton/KW scale. If they call a much bigger fraction then we can call BS
> When relativistic effects are taken into account, it is shown that the photon rocket can only reach energy breakeven as the accelerated mass asymptotically approaches the speed of light. Thus, any device with a thrust-to-power ratio greater than the photon rocket would be able to operate as a perpetual motion machine.
A photon rocket is 3.3 μN/kW, thus the EM drive is too efficient.