The primary concern with EMP is not the microwave burst that comes from the fission/fusion reaction, it is the DC-like current induced in transmission lines by the rising ball of plasma.
The problem is that HV Transformers are meant to take tens of kilovolts on their primary winding at an alternating frequency, which gives the oils time to absorb waste heat. When they are driven with, effectively, a rising DC bias, the thin wires in the winding will melt and burn out.
These transformers are not standard stock items and it would take months to begin replacing damaged units, assuming the infrastructure still exists to do so. We would be looking at trying to perform on-site rebuilds of these transformer windings with material that is available within a given region using personnel who are not familiar with the process… Assuming such processes exist.
Modern technology should detect this kind of scenario and disconnect the transformers to protect them - but the grid isn't uniform in its construction or upgrades - so there is a risk of cascades as stations fail and attempt to re-route through each other and begin overloading nodes.
It's one of those cans where you aren't entirely sure how risky opening it is, until you do so.