Since almost no one would be killed in the immediate aftermath of an EMP attack, a threat of retaliation against the aggressor country would lack credibility because such an option would be politically unpalatable.

CENTERFORSECURITYPOLICY: An urgent memo to the next government

Everybody now allied against him would have to freeze in place, and the victim country would begin the counterattack, trying to eliminate one aggressor country at a time by walloping each as hard as he could with the ball, beginning by throwing at those closest to him and advancing his position with each murderous thwack.

NPR: Roth Rewrites History with a 'Plot Against America'

Biological weapons, however, are often referred to as the poor man's nuclear bombs, and it is for the mass destruction of populations far removed from the aggressor's country and troops that they are most likely to be useful (and also for deterring your enemy from doing the same to you).

ECONOMIST: The whore of Babylon and the horseman of plague