After doing 3 attacks, electrocute doesn't go off immediately like Thunderlords did - there's about a quarter to half-second pause. It's really messing up burst combos - which is what it's intended for. It's allowing a larger window for shields and heals and whatnot, and sometimes it's interfering with Zed's ult, as the delay can cause it to proc after the ult if you're cutting it close.
Secondly; Luden's Echo doesn't count towards Electrocute, but Gunblade does. This may be intentional but it makes Electrocute feel very clunky, and removes one of the larger reasons to buy luden's in the first place.
Finally; With 15 available keystones to choose from, is it really unreasonable to suggest that an ability which strikes _twice_ should count _twice_ towards electrocute like it did with Thunderlords? I understand TLD was much stronger on some champs than others because of this fact; but it's a similar case with many keystones. That's the point of them. It's not unreasonable that press the attack is better on vayne than Orianna. So it shouldn't be unreasonable to suggest that Electrocute is better on Ahri than Xerath.