I've noticed that if I fight alongside allies, my ECM bonus only counts the ships I've deployed from my fleet which results in situations where I have a big numerical advantage with my allies (and I could have deployed enough to have a numerical advantage myself) but I end up with a range disadvantage. Now I am incentivized to let my allies die and then clean up after to avoid the range disadvantage which feels strange when I want to protect my faction and patrols. ECM bonus from the EW skill should include allied ships (or at least ships in your own faction) as well as your own fleet.
Something I'm also not sure about: when fighting multiple enemy fleets, if one has EW, do they get a bonus only for the deployed ships from the EW fleet or for all deployed ships?
edit: this also could become irrelevant with the skill rework, but I think it applies to any skill that has an effect based on what ships are in your fleet which should still be relevant.