Obviously Oda is a good writer.
His fortes imo look as followed
> 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