I've never seen Dark Harvest as good as Electrocute, but when i tried it with lethality Ezreal Jungle (Blind Pick) It was SO GOOD.
I got soo fed too, It's very good if you want to be a Hard Carry in late game. In my opinion this is the BEST rune for AD or AP, because it just like Electrocute but without a cooldown (except you need souls but that's gg if you're a jungle or laner) But in the end of the day, This is only my opinion, let me know what you think.
Here are the champs i use with Dark Harvest: Ezreal, Udyr, Lee Sin, Graves, Nidalee, Fizz etc.