Blackbeard took a Quake Punch to the head and a Bisento slash across the chest despite suffering extra damage only for him to go "Zehahaha!" The only time Blackbeard has been in a true pinch is against Magellan, a rather dubious scene when Oda was suffering hiccups in his writing style.
That slash did incapacitate him, and created the opening for Whitebeard to deal the Quake Punch. What would have the result been had he been able to deal a 2nd punch? We do not know, but it could possibly have turned the tides completely, but Whitebeard was simply too exhausted (
which is an understatement) to do any more than that.
Magellan's ability isn't explicitly more powerful than Whitebeard's Quake attack. It's not possible to compare an attack that works at microscopic biological level to an attack that works through force. Would be like trying to compare a dose of cyanide to the strength of a physical attack, they work too differently to be compared.
Poison is not force, it physiologically compromises any organism, even Whitebeard or any other elite Pirate would have fell if poisoned by Magellan, he just has an ability that no living organism is immune to. It's like fighting against sickness, no amount of brute force or strength could deter its effect. As we even learn, the Pirate King Roger couldn't defeat his illness, it's just the inevitable work of nature.
Caesar's ablity to create vacuums, anyone deprived off oxygen, no matter who they were, would lose consciousness. Magellan and Caesar's abilities are extremely powerful, they don't even need to be individually strong to poes a threat, their abilities compromise the body on a chemical level.
Personally wouldn't regard Blackberad falling to Magellan's poison as a hiccup, in terms of physiology and how biological organisms react, it's consistent that no one, no matter how powerful they were, should be able to defy the ramifications of attacks that compromise one's physiology and work at a chemical level.
