(or "The Nazis Were Bad PR")
It's a little unusual, reading the 1970s comic-book adventures of Doc Savage and the Fabulous Five; the character and his gadgets, methods and sidekicks are more firmly rooted in the pulp traditions of the 1930s than perhaps any other pulp hero of the era. (Which is why it may not surprise you to note that the Doc Savage doesn't even try to update the character, setting him instead in his own time.) The Shadow, the Spider, Zorro, Fantomas, Solomon Kane, Sailor Steve Costigan...they all have their pulp elements, and they've all made the transition to other media and endured the test of time (well, maybe not Sailor Steve Costigan...) But when you think of the elements that make up a "pulp hero", you ultimately wind up coming up with Doc Savage just as surely as if you're ticking off boxes on a checklist.
Pulp heroes tend to straddle the border between human and superhuman (Doc Savage has honed his physique and mind to the pinnacle of perfection with two hours of special exercises every day), they have elaborate near-futuristic gadgets (the Helldiver, the rapid-firers, the flying wing), they fight elaborate and outlandish foes (in the Doc Savage comics, he fights lizard-men, scientists who harness lightning, and fake pirate ghosts who work out of a high-tech mock pirate ship) and they work out of some sort of an elaborate secret lair (the 86th floor of the Empire State Building.) Even Doc's sidekicks are almost like "mini-pulp heroes" all their own, with one being a two-fisted chemist, another being a brilliant lawyer with a sword-cane, and so on. It almost feels like Dent took every single pulp trope and put them into the blender in an attempt to create a sort of pulp ubermensch.
Which is, in a nutshell, the problem with the character. He is portrayed, with absolutely no sense of irony, as a person who has improved mind and body to the point where he is considered by everyone in the series to be a sort of "superior being". Lesser characters question Doc's decisions briefly, if at all, before seeing that they're in the presence of a blond, bronzed superman who's smarter than they are and has superior judgment. The Fabulous Five accept his every word without question. Only Pat Savage, Doc's cousin and a superwoman in her own right, is allowed to suggest that he might be wrong about anything (usually about not letting her tag along.) He is every inch Nietzsche's ubermensch...
And that's a philosophy that hasn't aged well. The association with the Nazi regime served to highlight the uncomfortable truth at the heart of Nietzsche's ideas: It's very easy for anyone at all to decide that they are superior, have a higher moral understanding than others, and should not be subject to the rules set by "lesser beings". Every sociopath believes himself to be an ubermensch, and every murder improves the world in their eyes by removing those believed to be beneath contempt. (This is also the Republican economic philosophy, I believe...)
Doc Savage kidnaps criminals and lobotomizes them, he believes that women have no place doing dangerous jobs despite meeting women who are every bit as capable and competent as he is, and he ultimately accepts no input from anyone or anything other than his own moral compass. Generations of human experience have shown us that people like that are actually very dangerous and kind of creepy, something that limits the Doc Savage storytelling engine right from the get-go. Gadgets, sidekicks, and methods make a series, but it doesn't help if you suspect right off the bat that your main character isn't much better than the people he fights.