You're right, they are weaker: the Zombies are cheap war-labor who have a weakness that's not easy to deal with (salt). Keep in mind Luffy was beating up Zombies left and right, and still got overwhelmed, because they don't die, they just get back up.
Imagine what Marineford looks like if Whitebeard is physically incapable of dying unless he swallows enough salt. Good luck.
Also, we've seen Blackbeard be able to extract DF powers (IE lineage factor) from people (Whitebeard, Absalom). It's entirely possible that Blackbeard could give Zombies DF powers, the same way Vegapunk is capable of turning weapons into animals using Zoans. If a gun can turn into a dog, a Zombie can get DF powers, dang it.
God, how in the fuck did I end up in this forum shilling for Gecko Moria? Oi. I guess I just love his fruit that much.
I'd imagine Aokiji was the 10th Titanic Captain, but then either betrayed Blackbeard, or was kicked off, opening the door for Moria.
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Even when Green Bull isn't eating, he's eating, the absolute madlad.