My only problem with ECM is how it affects build diversity negatively.
What lights or fast mids do you see that aren't ECM?
Pretty much only the ones where people need to get them up to get their extra qualifications on the other mechs.
Same with Atlases, the differences in load-out simply aren't worth the extra benefits that ECM brings for FAR too light a cost.
As far as ECM itself is concerned, I like the mechanic, I don't have a problem with how it functions in general, my only problem is that it a damm over-powerful affect for something that costs the same as 1 small laser + 1 HS....
I don't' think there is any argument that its more powerful than anything of it's corresponding weight and size.
Simply put, ECM either needs to be equal weight/slots to the other equipment of equal value or at the very least bring it's own set of disadvantages with it, heres a few it could choose between:
Loadout Nerf - Ecm boats have less slots and/or very non-optimal slots
Power Drain - Ecm uses a lot of power, reduces max speed, turning e.t.c. Weapons reload slower e.t.c.
You can see what? - ECM is putting out a load of interference, unfortunately interference doesn't really understand friendly fire so the ECM boat is jammed itself while disrupting (Not countering)
You can see me! - Well, the ECM boat is putting out a lot of transmissions, even if its masking it's friends, it shouldn't actually be hard to get at least a rough location on the jammer itself. (Also added a way to turn it off making ECM usage more tactical)
Mega-Death - You know how capacitors go bang a lot (like in Gauss) when shot, well the type of capacitors ECM would need could easily short from EMP... BOOM! ECM boats go bang like Gauss
Sensitive Equipment - Lets face it, broad spectrum radio transmitters aren't exactly durable. Every hit to the location of the jammer could do it damage (possibly risking explosion?) that could disable or destroy it or at least reduce it's range, even when the Mech has armour left.
Or we could introduce HARM.
For anyone who doesn't know, HARM stands for High velocity Anti-Radiation Missile, basically when there is ECM about, you fire it up into the air, it looks down, goes 'Oh look, I can detect a lot of ECM from there' and heads straight for the nearest active ECM source, ECM source either turns it's ECM off before the missiles hit or takes the damage, they pretty much have a lock as long as the ECM is turned on.
These also work on Radar sources but since the only thing that uses active lock-ons in the BT universe are missiles in flight, they wouldn't be too much use there