Spanning over many years, the story doubtlessly deviated from its original course (relatively quickly anyway) and Samura didn't care much for world build or history, but I do not think these two aspects are truly relevant. What really matters in this work to my eyes is the exploration of violence, revenge, hate and related themes. On that front Samura does a fairly good job. To be honest, it's nothing really that deep or profound. But exactly because it's rather simple and elementary, it allows to be understood easily by anyone. And even more than the themes themselves, what really makes the greatness of this manga is the way they are expressed, so in one word the art. Yet I am not praising just the art in itself (which, mind you, it actually is already great in itself, if you ask me), but rather the way Samura uses it to express simple and yet never exhausted themes. This work to me is somewhere in-between a proper manga and an artbook. The story is, as often is the case in many manga, little more than a pretext for the author to portray certain scenes. The fact that Samura doesn't care much for historical accuracy only goes to show his purely aesthetic reasons for choosing his setting.
It's worth noting how Manji, the supposed MC, fades into the background more and more as the story goes on, and he turns out to be more of a spectator than an actor of the story, which pretty much suits his existence as an immortal that is something other than human, and this sense of alienation is further reinforced in the final chapters. His main playmate is Shira, which in body and mind is a freak just like Manji.
The greatest expression of Samura's art in this work is probably Makie, a triumph of violence, coldness and sensuality, mixed in with a sense of ephemerality and caducity, faced nonetheless with a will of steel that accepts mortality and misery, and yet is not willing to give in until the very end.
Kagimura and Anotsu are two others great characters. Anotsu is unable to accept the contradictions of what happens to the martial arts when they are reduced to a mere formal system that cares more about etiquette than efficiency. His Itto-ryu has only one single rule, and that is the prohibition of overnumbering the opponents. This differentiates it from being a mere no holds barred, since those who fight without rules only care about the outcome while those who abid by the Itto-ryu's rule are fighting mainly against themselves in a battle of self-improvement. What Anotsu care about is the personal growth in real battle situations. A very down to earth and elementary, even primitive, drive: the desire for true personal power founded on one's own ability rather than depending on other people. Yet a desire that contradicts the very nature of what human society has become, and hence Anotsu wishes to change that same society and show to its the value of his pursuits, to show them how weak their illusionary power dependent on technology and bureaucracy actually is. And of all this Kagimura, the warrior who became a bureaucrat himself, is well aware and what drives him the most in his hunt for Anotsu is not the orders he receives from above, but rather his envy for what Anotsu is trying to accomplish, something that Kagimura desired in his heart of hearts but never dared to pursue. Though ittedly Kagimura is conflicted, because he still sees value in the society that Anotsu wishes to destroy. That's why he ultimately chose to sacrifice his own personal pursuits as a warrior for the sake of order and stability, for the sake of his family.
There is no black or white and there is no clear answer in the end. Even knowing the pointlessness of her own revenge, Rin can't help but wishing to go through with it nonetheless; even knowing the pointlessness of his social struggle, Anotsu doesn't put down his sword until the very end; even knowing the pointlessness of his battle against the already fallen Itto-ryu, Kagimura doesn't stop chasing Anotsu. The feeling that we get from the whole picture is that of a human life made up of things that are cherished and that are ultimately lost, yet we can never let go of them even if it's hopeless to try to grasp them, and so we keep fighting a losing battle not for the sake of the future but rather for the sake of the past, because if we gave up everything we had done up until now would lose all meaning.