Honestly, I liked this series. It got axed and ended really badly because of that, so let's set aside the ending.
Science-fiction in manga is not that common, and I think Samurai 8 blends the scifi elements well with more traditional spiritual elements. The scifi setting brings to the table this 'vastness of the universe' feel (that unfortunately didnt live up to it's potential due to the short run) and also this incomprehensibility of the powers at be. Of course, this is one of the reasons that scifi is unpopular in manga. Typical shonen mangas have very rigid power systems which is cool for intense battles but boring for worldbuilding.
Samurai 8 also appeals to me esthetically, though this is not something I feel like there's much to argue about. I like the character designs, I like the blend of old and new, I like the backgrounds. And of course the execution of it is quite good, coming from a veteran mangaka.
Overall, there is some valid criticism of this manga, but I really wish they let Kishimoto cook. I wanted to see his vision. Now he's stuck with endless naruto spin-offs and we'll never get to see what else he was capable of. I wish he'd been given some more slack.
btw this was written quite some time after I read this so idk i might be completely off the mark