Tldr: ECM makes your locks go kinda fuzzy.
ECM stays chance based, you don't lose your locks but your targets get harder to damage or otherwise affect.
Ratio of ECM strength vs sensor strength determines the penalty to the ECM'd ship, stacking penalties should apply.
Turrets on an ECM'd ship would get an increase to the signature stat, missiles an increase to explosion radius. (or if it's easier to implement a decrease in the sig radius of the target)
Webs, sensor damps, tracking disruptors, painters, nos, neuts, remote reps, remote sensor boosts, cap transfers, etc would be less effective.
I'm not sure about scrams and disruptors. I'm torn between less range or still get shut down (either way effectively ECM still gives you the GTFO ability)
I would theorycraft some numbers, but effort...