The problem is that at high enough values for exhaust velocity and thrust , the amount of watts in the jet is too much. "Too much" is defined as: if only a fractional percentage of those watts are lost as waste heat, the spacecraft glows blue-white and evaporates. The size of the dangerous fractional percent depends on heat protection technology. There is a limit to how much heat that current technology can deal with, without a technological break-through.

Conventional nuclear weapons will also produce an EMP if detonated near an atmosphere. They will not create one in the vacuum of space, with no air molecules for Compton Scattering.