Nobody really uses much undirected EM warfare in my opinion because it represents a huge escalation in a war, similar to the use of indiscriminate chemical weapons, or even nuclear weapons.

It would be devastating in the local battlefield, potentially frying radio or other equipment depending on the size of the device or how close you could lob it towards the enemy before going off; but with the low wattages many non-military communication devices use today you would also be blasting horrible noise to all of them beyond the local area and disrupting communications across potentially multiple neutral countries.

It would be a large act of aggression against any countries around them and NATO, and at scale possibly even piss off far away countries like the US and China. Especially large EMP devices could even be temporarily misidentified as a nuclear explosion and gain the immediate full attention by any nuclear powers watching out for it.