How Op is DH really?

I been watching the forum since pre season released and everyone has been going nuts over DH. I seriously can't see how DH is as good as everyone say it is, after all it only procs at 50% HP has pretty bad scaling (outside of the proc scaling) and the base is about the same as Electrocute, (a bit less after nerf). But people are still going nuts about it. I'm still using electrocute on most burst champions. I can see it being good on ADCs and junglers, but why go DH on an assassin/mage over some of the other spells? I can see it being better on champions like Teemo and Jayce who usually goes aery but otherwise I don't get it.
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