I've seen people saying it's too easy to proc and use and anyone can use it. But doesn't that make the rune good?
Runes are supposed to be usable by anyone at least in theory. What's the point of having a rune that only works on 1 champion (extreme case)? Why not just add that functionality to the champion itself?
I think more runes should be a bit more like Dark Harvest in that pretty much anyone can use it. For example, Spellbook is another great one.
I think Electrocute, on the contrary, is a bad rune. It's designed so that a limited number of champions that can effectively proc it, can use it. Same with things like aftershock.
Why not just add Aftershock functionality to Leona and a few other champions and then remove the rune? There's actually no generic defensive keystone which is really silly.
We need more runes that can be used by anyone!