Thank you. Yesterday I finally had a chance (3 BR7 Dogfight Battles) to try it... and it was very cool :) . It was a 2-2 players room, so my teammate sometimes was out of my range but I think (hope...) the ECM helped him too :) . I think the EA-18G teleported on the list of my favourite planes :) .
I can image how useful can it be in Raids (Emergency xyz Strike...) :) .
ECM works in all missions in the game. However, it only reduces the risk of enemy missiles. It doesn't change anything about enemy gunfire, which is almost absent in Air Strike but is highly prevalent in Sea Strike and Ground Strike.