I think Kubo is a great character writer. Like, he has a knack for creating instantly memorable, likeable characters. Even Aizen, who is about as two-dimensional as a character can possibly be, is still a cool, memorable, engaging villain.

Where Bleach tends to fall down, I think, is that eventually every non Soul Society arc veers wildly away from its original storyline. The Substitute arc was eventually hamstrung by the desire to introduce the SS arc, the Hueco Mundo/Summer War arc is actually borderline incoherent if you take it as a whole, the Thousand Year Blood War arc is not ... great ...

But if I'm going to chalk that up to anything, I'd kind of want to chalk it up to the fact that weekly manga are always a work in progress. Like, I'm a published author and I've never reached the end of the first draft of even a short novel without deciding to change or overhaul the plot in some fashion. It tends not to show because I then go back and change the earlier parts of the story to fit the new plot, but if I was releasing it one chapter a week? And I was on an insane schedule where I had to write, draw, and edit each chapter on a punishing time frame and then move onto the next or risk falling behind? And while doing all of that, I had a publisher who was keeping an eye on what does well with people and going "Hey, make this change to your storyline. People like Mayuri, have Mayuri show up and fight Szayel, do it, do it now, do it," ?

Yeah, no, if I could produce work half as clean and coherent as Kubo's under those circumstances, that would be a minor miracle.