No one really knows.
Testing EMP effects is extremely difficult.
Real world tests of EMP are nearly impossible because the effects are so widespread.
Survivability of devices exposed to EMP, on what small scale testing has been done, varies enormously depending on whether the device was turned on, plugged in, or shielded.
An EMP attack is essentially a nuclear attack, and should have the same consequences for the attacker.
Risking nuclear retaliation with an attack where the attacker does not know, and cannot know the actual effects, makes an EMP attack less likely.
There is a huge difference between an EMP which destroys/disables 75% of the power grid and communications systems, and an EMP which destroys/disables 99% of the power grid and communications systems.
That is probably true but humans are more easy to predict even if the electrical grid was 75% destroyed it would create a situation where anarchy would rule and humans would start act irrationally those with the guns would be able to defend themselves otherwise they would become victims