Are people really downplaying Blackbeard? No clue why people think he is going to have weak Haki.Remember guys Haki is simply will power manifested and we've seen that Blackbeard obviously has incredibly lofty goals of ruling the world or destroying it. Having two devil fruits isn't going to make his Haki any weaker or make him less dependent on it, it's his ambition in the first place that enabled him to steal both of those fruit in the first place. He also was offered the position of Commander of the second division but turned it down (Garp style) to stay under the radar.
He's a complete monster and will continue to be exactly that, Luffy is going to have a difficult fight with him, but I'm guessing that Luffy will be resistent to the effects of the Earthquake fruit considering he's rubber, but if Blackbeard grabs a hold of him that's going to be a nasty feeling having everybone in your body shaken to dust.
I think it's because of how unusually Blackbeard has been presented. Obviously Blackbeard is extremely strong but he is not portrayed as the usual super powerful characters. He takes and gets hurt by attacks that any character in his power level should be able to easily dodge/block. He got obliterated by Magelan, a half dead Whitebeard beat the crap out of him to the point where he was cowering on the ground begging to be saved.
So yeah I think subconsciously a lot of people will keep underestimating Blackbeard's power until he gets his classic badass moments