The EMP is a great ‘first strike’ weapon.
During a possible conflict, any site capable of launching such a weapon would be destroyed during the fueling stages. This is a weapon you launch to start an offensive; not after a conflict has been engaged.
That's nice. How?
If the Norks wanted to first strike us, all they'd have to do is tell us the rocket they were about to launch was "just a harmless test". They launch it...and by the time we realize what they've done, it's too late. POP! and the entire electrical grid, as well as all the computers and fancy-dancy electronic stuff we rely on...gone.
First strike? It is a single strike weapon. The grid goes down and there is chaos. It would take years to recover.
Life expectancy would drop to 1850’s levels.