Shin and Lu need a bit more strength/skill.
The series is fantastic, but we already have enough characters to babysit with Sakamoto's wife and daughter.
If Shin, at least, gets closer to Sakamoto's level soon, and can fight for longer periods of time, the series will get a lot better.
That's all I have for criticism, really, though, the fight scenes, characters, and world building is all top notch art. Some of the most kinetic action I've seen in Manga, you can feel a lot of the force in the violence. It doesn't make assassination look like fun, it makes it look badass, and crazy.
Still can't believe the deer guy, Kashima, survived all that shit.