This one is between a 7 and a 6, but its visual hindrances stop it from transitioning into something worthy of major praise. Saying that, I very much think that the show, as a whole, has quite an accomplished story and a bunch of well-realized characters. It is underrated, I cannot stress that enough. Manji and Rin, our MCS, have a great dynamic, and besides them, there are a lot of others who serve their role well. Magatsu is a lovable warrior and a fitting rival to our main man, while Shira is a pure menacing evil. Kagimura's mustaches speak enough on their own, and many other colorful characters constantly refresh the action and give us many moments worth of remembrance. The key to this ONA being special is the directing, with a lot of experimenting that works most of the time, nice transitions, beautiful painterly animated movements, and action done in a cinematic and kinetic way. Although it came in 2019, in a lot of ways, it has old-school (90s/early 2000s) anime qualities. There's much more attention given to designs and strong drawings, while the fluid animation is used sporadically when needed. The music is also truly amazing, featuring faster jazzy tracks and traditional Japanese instruments in the slower ones. That is how the whole thing works. When there's carnage, the heads are flying and there are many cuts per scene, but there are slow contemplative moments too, where the "camera" lingers and the scenes breathe, full of atmosphere and buzzing dragonflies.
If it seems like I'm singing too many praises, I think the anime deserves it. The bad bits are as I called them, bits. The animation has those problems, it is rarely superfluid or incredibly impressive, but it's good enough. The directing elevates those scenes in a way that doesn't make them boring or straight-up bad or ugly. There are some pacing issues as well, which is to be expected when you adapt 200+ chapters into a mere 24-episode series. But the pacing in itself is A-OK, it's just when you compare it to the manga that it seems bizarrely fast, adapting sometimes a volume in an episode. But hey, look at the movie adaptation, it succeeds in telling the essence of BOTI in a 2 and a half hour movie. The series, succeeds as well, in telling that same story in a series format, simple as that.
Would it have been better if the anime got a better studio (sorry Liden Films), a better schedule, and a more talented crew of animators? Yes, but for what it is, it's a damn good time. The water wheel keeps spinning.