Is Danny Jacobs a bigger threat to GGG than Lemieux was?

Jacobs is a far more dangerous puncher. Now does he hit harder? Perhaps not.

 

How is GGG able to hit these fighter on the top of their heads with out hurting his hands.

 

He never uses his fingers and curves his punches so that he uses his folded fingers.
Here is an example:

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Look at how he hooks his punches so that the full pressure is on his folded fist and not his knuckles.
In the Stevens fight GGG had to dig deep to finish. People have ADD nowadays so they have selective memories but Golovkin had to pull out his "A" game in this fight. Check the curve on the hook to the head.

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Now just as a counter look at how Mayweather slaps is hooks against Canelo. Because his entire style is speed/defense based he never commits to punches like he did under Roger tutelage

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This is why he broke his hand or hurt it vs Guerrero too.

 

Lemieux is garabage and always has been. Jacobs is way better. With that said, I'm not sure if Jacobs is much better than Brook at middle. It's hard to gauge well Brook fights at that weight with only one fight against the best guy there.