Oda is very good at designing characters and places on a superficial level. His wordplay and cultural references are outstanding and he excels at creating iconic and memorable moments in his series.
Where he suffers, however, is in logical consistency and stringing his ideas together. He wanted Sanji to make a cake. He wanted to reveal Sanji’s family and extended backstory. Ultimately delivered in neither aspect with an unsatisfying ending to the Judge/Ichiji/Niji/Sora/Sanji familial bond and an utter failure of a cake plot line that underwhelmed fans and... I would say it underwhelmed Germa too except they never saw the damn thing. Oden carrying his retainers and marching to Orochi’s castle are beautifully drawn. However the reasoning behind the plot line is incredibly weak and there are too many confounding elements for the mature, critically minded reader to enjoy the story fully. Logic went out the window in WCI and seems that Wano is no different. Better to deliver the basics of Shonen (cool designs, cool moments and fights) and do it well than to attempt to be too creative and fail.
The sheer breadth of story at this point is pretty noteworthy. There are other long-running "universe-building" fantasies that have grown so broad/messy over the years that certain stories/arcs are now "no longer canon," like Star Wars and Dragon Ball. In that regard, Oda has managed to keep things impressively tidy when you consider the number of major subplots he's running at this point b/w World Government, Celestial Dragons, various pirate groups, ancient mysteries, Blackbeard, Kaido, the Revolutionary Army, etc.
I have some skepticism/anxiety about how things ultimately resolve, but so far, it's still pretty engaging and I think you have to admit he's done a terrific job of maintaining the humor and weirdness of the series throughout all the melodramatic plot twists.
Either good way or bad way, he makes you lose mind most of the times and discuss about it and that is what makes him quality writer......
My only qualms with him is that he stretches unnecessary things sometimes (like WCI second half, G4 cool down, Udon training etc. ) and off panels or rushes important stuff (like WB vs Roger fight etc.) or makes very important characters stick to background (Toki, Robin etc.)
He's a good writer, and One Piece's length is a testament to that. It's just that Oda wants to end the story soon, but there's a lot of loose ends and plot points that he wants to include. It's making the recent arcs feel overly lengthy and complicated. Dressrosa being the best example. The writing was a little tighter and more focused pre-timeskip.
He's a good writer overall but he's not without his flaws with how long he can drag an arc like he did with Dressrosa arc, his overuse of plot armor and of ridiculous plot devices such as Mother Carmel's picture, Habuka, Big Mom's mental crisis and later amnesia or Oden's dance and his very questionnable use and treatment of female characters like Tashigi, Rebecca, Compote or Smoothie.
> Good planer, the fact he conserves his panels despite his manga running for multiple decades allow him to come back to them at any point of his story, which isn't what many writer pay attention to
> Foreshadowing, because of the above, it allows him to reintroduce events that he wrote decades before and thus making it look like an awesome foreshadowing
> Drama, Oda is very good in writing drama events, to portray evil, despair from the innocent people until the hero comes and save the day
His weakpoints imo:
> Too much focused on Luffy, and because of this, it makes him off-panel even important events that have been hyped since a long while like the reverie, with the battles between the revos and admirals only brought up and conclusion mentioned
> The above could also be seen from his difficulty to focus on multiple events at the same time. Some mangas like HXH can have multiple chapters and volumes focused on multiple events scattered in multiple different areas on the arc or the manga itself which imo gives more life to a manga, but One Piece doesn't do that
> Battles, most one piece battles consist of few hits spread out from chapters to chapters while being skipped by dialogues or events outside of the battle situation. There are few exceptions to that like the Luffy vs Lucci battle which imo is still one of the best one piece battle to date(which shows btw that Oda can easily do that or at least could), but they are so few, it makes battle perspectives less entertaining, and sword battles imo are some of the worst drawn in mangas(if you read other mangas like OPM, you can easily see the difference)
With that said, Oda stated himself that he decided not to focus too much on battles because his manga would have easily been beaten by Dragon Ball if it was the case, and thus decided to write a manga as dramatic as he could to attract more attention, which was very successful for him for decades. The story around pirates was interesting since the start, with the involvement of marines opposing them, Shichibukais as third power, which indirectly lenghtened and increased the manga potential played a huge part into this too
Imo, the Supernovas introduction in Sabaody was promising too, but thinking back to it, i think he introduced them too late. He should have introduced them at the beginning of paradise to give them all significant roles, before starting to create a difference in the new world. Either this or he should have reduced their number in half like he stated himself, because we're already entiring the final phase of the manga and most of them have yet to be properly developed. With that said, Capone's portrayal since Zou pleasantly surprised me, especially since he was some of the one with the least portrayal preskip and thus gave me hope he could still manage something huge with them, especially after Morgans' final speech concerning them
So overall then, i'd say that Oda is obviously a great author. However, just like every humans, he has his flaws, but it doesn't really change the fact that One Piece's story caught our attention for multiple decades and will continue to do so, which in itself is to praise, so while there were some drop in quality here and there, i still have good hopes for the one piece manga for the future to come. The throne war battle between Yonko, Shichibukai, Worst Gen, Marines, Revos should be exciting
To create a series that sits on the top for 20+ years is outrageous.
Many manage to do it for one year, when the hype is strong. Some manage to it maybe for five years. Very very few can do it for ten years. Doing it for 20 years is unbelievable.
His story has much more layers than most other series, his symbolism is top notch and his character and world building is great.
He has his flaws, too (fights, sometimes getting too creative, pacing at times), but everyone has flaws.
He's a good writer, One Piece's success and length is a testament to that. I think his absolute best aspects are world building, character design, cultural references, foreshadowing/planning ahead, and powerful character moments. He does have issues though in my opinion, for one he's not very good at writing fights. They're mostly lackluster trading and tanking attacks, and whoever has the most stamina/willpower wins. There's rarely any creativity or intelligence in the use of devil fruits and there's little to no tactics involved in character's attacks. Also Haki as a system still has some inconsistencies that need to be ironed out by Oda. I'm not saying fights are everything, but in a shonen battle manga I would like to be entertained when fights do happen and I'd like the win that the protagonists get to feel earned.
Another issue Oda has is the formulaic nature of arcs and characters. Most arcs usually involve arriving and getting a feel for the new setting, they come across an oppressed person, main villain is introduced, Strawhats vow to help, very melodromatic backstory usually involving the death of someone close to them, strawhats fight and defeat villain, they're hailed as heroes, rinse and repeat. That's not to say all arcs are like that there are some exceptions, but for a story going on this long that format is in a majority of the arcs. Oda also has issues of in-arc pacing, he will cover something in 3 chapters that can easily be condensed into 1, a lot of Dressrosa and Whole Cake Island come to mind. I remember doing a reread of Dressrosa and near the end when Luffy needs to recharge his Gear 4 two of the chapters are almost exactly the same back to back, just screaming everyone to hold the birdcage felt like Deja vu reading it. Final issue is a more personal one, but I'm really not a fan of how Oda writes women. If they're not eye candy or fanservicey out the wazoo than they're almost always a damsel in distress. If they don't have either of those than their characters are usually for comedic effect. I mean there are a lot of great female characters in One Piece, but Oda doesn't develop them in nearly the same way he does male characters. So yeah overall Oda is a good writer, but I do have issues with some things.