My understanding is that they didn’t have any effect because the electronics were not as sensitive as they are today.
The electronics were not as sensitive then but there were effects noted. Those effects are what led to the discovery of EMP.
Our military has a EMP generator that can be loaded onto aircraft to use as a weapon. If EMP isn’t real why would they build a weapon?
http://www.digitaltrends.com/cool-tech/us-air-force-confirms-boeings-electromagnetic-pulse-weapon/