I have been paying attention to this lately. I'm close, but I do not have any legendary mechs yet. But, using my epic mechs, I noticed that any stuns I use do not take effect right away on them. I do use a Zephyr in my hanger. I have used the shock pulse quite a few times. But everytime I use it on a legendary mech, then get like a second to be able to attack me. Using it against a Dreadnought, the mech will stop moving right away, but they can get off an attack after they stop and more often then not end up killing me. It just seems like 99% of the time they still get to attack me after they are stunned. Don't ask me to send a video of it because I'm not set up to make videos from the game, and screenshots won't do anything to show it.