The line on Dororo どろろ, which Tezuka Osamu 手塚治虫 serialized from 1967 to 1969 in Shūkan Shōnen Sunday 週刊少年サンデー and then Bōken-ō 冒険王, is that it was an attempt to compete with people like Shirato Sanpei 白土三平 (by writing a dark, bloody samurai piece) and Mizuki Shigeru 水木しげる (by peopling it with traditional and traditional-seeming monsters), and that it was a failure.
The story revolves around two main characters. One is Hyakkimaru, a youthful swordsman who was born with basically every horrible physical handicap one can imagine: no arms, no legs, no eyes, no ears, no nose, etc. This is because his father, a lord of samurai, had made a deal with a collection of 48 demons that he'd sacrifice his newborn son in exchange for the power to conquer Japan. Each demon, he said, could take a part of his son. And so they did. The boy survived, but his father cast him adrift in a basket on a river. The boy was found and raised by a kindly physician who made artificial limbs for him, with special effects: he could take his arms off to reveal sword, he had chemical weapons hidden in his fake leg, etc (yes, shades of Mighty Atom). To boot, the boy learned that he could compensate psychically for his lost senses - talk, see, smell, hear, etc. mentally. When he sets out on his own, the boy, known as Hyakkimaru ("Hundred Demons"), learns that he has a special talent for fighting and killing monsters, and that each time he does, he regains one of his stolen body parts. So he's on sort of a quest to rebuild himself by conquering all the monsters.
Early on he gains a sidekick, a little boy named Dororo whose parents were Robin Hood-type populist robbers betrayed by a greedy sidekick. Dororo is an orphan, and a little runt, but he prides himself on his burgling abilities, calling himself the best thief in Japan, and sets his sights on Hyakkimaru's sword (not the one in his hand, the normal one he carries at his side). Thus they team up to travel the land and fight monsters...
It's not a bad setup, and Tezuka is a natural storyteller, so the tale never lags. It's always interesting. But. It has several fatal flaws. One of them is Dororo himself. Unlike Hyakkimaru, he doesn't really have a story arc. He has a back story, and it provides some hooks for further episodes, but it's never clear what Tezuka wants to do with him as a character. And, more importantly, he's just annoying. He's a standard-issue spunky boy hero, which is fine in some contexts, but here he's as cloying as the little boy in the second Indiana Jones movie. He's out of place here - his presence shows that Tezuka either wasn't fully committed to telling an adult story like his competitors, or just didn't know how.
yōkai that Mizuki does. The story feels derivative, and more than that it feels jokey toward its material in an inappropriate way. Part of this comes down to the art: Tezuka famously never quite outgrew the Disneyish character-design on which he had built his career. Much later than this series he finally managed to temper it enough to enable it to tell serious stories, but at this point he's still all-out cute. There's a disconnect here, considering that he's amped up the violence to compete with Shirato. Seeing cute li'l Dororo cavorting amidst all that blood'n'guts is frankly a little disturbing.
But it's not all a question of style. Like I say, it's also a question of feel. Mizuki took the idea of yōkai and used it as a way to explore mood and atmosphere: even with all the gags in his Kitarō series, there's an amazing variety of creepiness afoot. And of course it goes beyond that: he uses yōkai to satirize the present, preserve the past, suggest a whole alternate take on Japanese cultural history... In Dororo, Tezuka seems to see yōkai as nothing but scary obstacles for his heroes to overcome. His monsters don't have the extra dimensions that Mizuki's have.
The same can be said for his treatment of the medieval setting. Given Tezuka's vaunted humanism one might have expected him to find a lot to love in the ninjō heavy conventions of the jidaigeki. I'm not sure I can put my finger on why it doesn't work, but his samurai are one-dimensional, and so are his peasants. His treatment of the feudal social system feels preachy, not rich with ambiguity like Kurosawa's (another of his obvious models here - the tip-off is an early scene in an abandoned temple gate strongly reminiscent of the one in Rashomon).
Tezuka has, of course, a towering reputation in manga, and so it's worth reading any of his work you can get your hands on. I've read all or parts of six or seven of his series - not a big proportion by any means - and I have to say, I find him kind of hit or miss. When he's in his comfort zone of kiddie adventure comix, he's untouchably brilliant. When he strays out of it - not so much. Above I mentioned the Indy Jones movies, and in fact I've come think of Tezuka as being a lot like Spielberg: a genius in a field that was at the time not well respected, who pushed himself to work in well-respected fields to mixed results.