It breaks the laws of physics as we know them. But we don't know everything, there is still a remote chance.
Conservation of momentum is so well established by this point, that's it's a very remote chance, and I'd expect it to still be conserved in that case, just through a mechanism we did not anticipate.
I'm just really rooting for new physics here, because I think is been about 70 years since we've had that.
Conservation of momentum is, via Noether's theorem, a mathematically proven consequence of translational symmetry in uniform space, i.e. that physics is not affected by linear movement through uniform space.
The bar for violating that is super high, unless you don't believe in mathematical proof.
Of course the space we occupy is not entirely uniform, due to general relativity, but GR addresses that, and the results are not consistent with em drive results, i.e. momentum is still conserved.
Conservation of momentum is so well established by this point, that's it's a very remote chance, and I'd expect it to still be conserved in that case, just through a mechanism we did not anticipate.
I'm just really rooting for new physics here, because I think is been about 70 years since we've had that.