[antonvs]

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.