Samurai 8 is awful. I am honestly very disappointed with what Masashi Kishimoto has come up with, several years after finishing Naruto. I just finished the manga, I had dropped it last year, about 32 chapters in, but now I decided to come back and finish it, and my impression of it is even worse than what I originally had.

This manga had a lof of potential, it could have been a good adventure with compelling characters. Many of the characters introduced have varied personalities, but unfortunately the story never really did anything with them. I'll start by saying the two things that I enjoyed, and that was that the battle choregoraphy was good, it reminded me of Naruto. The art is also interesting, even though it doesn't start off great, specially the fact that it didn't have enough shading, but with time it got better.

Pretty much everything else is bad. There is so much wrong with this manga, that Kishimoto failed so hard at. It's funny because one of the biggest complaints about Samurai 8, the fact that there is an excess of information and exposition in every chapter (starting with the 1st chapter), was adressed by Kishimoto himself before the manga had even started serialization. In an interview he had said how it's difficult to write a sci-fi story for the Weekly Shonen Jump audience because sci-fi usually has a lot of terms and knowledge that needs to be conveyed to the readers, and so he talked about the importance of timing the exposition properly and not having too much information crammed into a single chapter.
And then he proceeds to do the exact opposite of what he said... the very first chapter introduces a lot of confusing terminology and all the chapters follow up with more, and it just gets very boring very fast because much of this information is either unecessary and excessive or just poorly explained.

Another thing that the manga horribly fails at is the power scaling. It feels like every 5 or so chapters Kishimoto changed the power system and nothing ever made much sense. It begins with introducing the trinity, and how Samurai are powered by princesses and key holders, then Samurai can regenerate their whole bodies after taking damage. They also cram in a bunch of techniques that are poorly explained and half the time it's difficult to even understand what they actually do. Then there is the plot armour of how the "chosen samurai" have stronger white coloured samurai souls that can cut through everything, but only when they are feeling brave, and they keep adding to that, with how samurai will never die "as long as they are courageous enough". Samurai can also get instantly stronger if they have princesses praying for them, but that works awfully inconsistently as well. Finally, about 8 or so chapters left for the manga to end, Kishimoto introduces an RPG system where each Samurai has stats, and in order to use certain skills they need to have certain stats. Unfortunately they don't even bother explaining what most the stats mean, so it just feels like it was something that the author came up with at the moment but then didn't have time to finish. It would have been better imo if the stats were introduced at the beggining of the manga and they had started the whole power system with it in mind.

The villains in the manga are also horrible. There is one minor villain that appears in the second arc, but he's just fodder for the main characters to beat up, he doesn't really have any motivation or anything, he just wants to make money and kill people for fun and power.
The main villains unfortunately are not compelling as well, their objective is the cliche "destroy the universe and rebuild it anew". It honestly doesn't make any sense, and there are several things that Kishimoto ddin't bother talking about, like certain characters that they teased in the first arc but then they never showed up nor were mentioned again. The villains' backstory was never broght up as well.

But then the finale of the manga is when things get really, really bad. They introduce a bunch of characters that don't serve any purpose, it really feels like Kishimoto was starting a new arc but then he was notified that the manga was getting axed so he had to come up with a finale at the last minute, and it was awful. So many events are crammed in the last few chapters. The villains suddenly show up and fight the heroes, they use a bunch of unexplained techniques like trapping characters in a void and then, when it seems that everything is lost, a character shows up out of nowhere, and now for some reason they're a samurai, and after a cheesy dialogue the main character reaches enlightenemnt and becomes ultra godly powerful for no reason. Then they're able to get into a dimension where space and time act faster or whatever and so they age and become ultra mega powerful and one of the villains gets killed without even getting properly introduced.
The last 3/4 chapters are such a gigantic mess that they alone would be rated by me as a 1/10. The overall manga gets a 3/10 for me because the rest of it is not as bad, but I'm definitely extremely disappointed with how the manga turned out. I feel like it had potential, and some interesting ideas (that were poorly executed), but the result is abysmal.