So! I'll preface this by saying that I have high hopes for Sakamoto Days to improve, and will be doing my best to not be a complete negative nelly.
Now, to begin....
Sakamoto Days IS a good manga, it has a lot of good things to it. It's not amazing, it's not groundbreaking, it's not an opus. It's a newbies foray into the world of writing, but as it continues on it feels as though someone, whether it be the author himself, or an editor, is trying VERY hard to push it towards being something it simply is not.
Which is to say; A heavily plot focused and story driven series.
You may be asking yourself "Isn't modern manga supposed to be plot and story driven, though?" to which I reply... Yeah, I guess? But not every concept, not every story, has to be a huge spectacle. A story can be as simple as the initial Sakamoto Days concept. An ex-assassin, the best really, being a domestic man, and the daily lives of he, his family, and his adopted family.
Sakamoto Days does it's slice-of-life EXTREMELY well, and excels in goofy humour that takes advantage of the inhumanity of Sakamoto and Shin, playing off of the unreality of their abilities to create fun and interesting gags that you don't often see due to Shounen favouring not making fun of it's main characters the way Sakamoto Days does.
Sakamoto is a great character. Like I'm sure many people would say, he is comparable to Saitama of One Punch Man. Overly powerful, and seemingly indestructible. However, unlike Saitama, he has a particular softness to him, showing that he truly only wants to live quietly with his family, the people he loves and would do anything for. Sakamoto cares, and he cares deeply. It's endearing and interesting to have a main character like him, who is already strong, and already has his priorities wholly in order. He's still very much an assassin, though, and shows his violent tendencies to Shin and Shin only in a running gag that hasn't gotten old!
And Shin is a fun foil for Sakamoto, often playing the straight man to Sakamoto's violent humour-style. He's earnest, and a bit on the awkward side, with some meaner habits. But all of this just allows him to play a role he fits well.
I'll skip over the main side characters for the moment, but may return in the future to put out my thoughts on them... Though I will say I would end a man for Heisuke. Love that man so much. Good boy, best boy.
As I said before, Sakamoto Days does its slice-of-life and more down to earth stories and one-offs best. And that draws us to what is making me enjoy it less and less as the chapters keep coming...
The Actual Plot is Sakamoto Days' Weak Point.
Completely bluntly, honestly, the plot is not paced well. There has been little-to-no foreshadowing or build up for the (current) antagonists, or for the assassin side characters that aren't the Sakamoto Family( Sakamoto, Shin, Heisuke, Lu ), and it is SEVERELY hampering the writing, and my own enjoyment.
Everything feels like it's going at a rapid fire pace currently( chapter 28 ), trying to rush through what should be it's first proper arc, seemingly in order to present 'Cool' characters who aren't as cool as the author might want them to be. With no build up toward them, to their personalities, to their ties to the grander scheme of things, they are little more than cardboard props being sent out to fight the Sakamoto Family. They have interesting concepts, and abilities, but with nothing to support them, they feel.... Non-threatening. Uninteresting. Flat. Dull. They aren't characters so much as mildly neat ideas shoved out without any care to their longevity or appeal to a fanbase.
And then the Other Assassin characters... The ones who I quite literally can't remember how they're related to the story due to how little impact they made despite them obviously being important and, again, attempts at making "Cool" characters. Since I almost entirely forgot they existed, I can't say I'm a fan of.... Whatever they have going on, which certainly isn't interesting, or giving them any personality beyond being "Cool".
As you may have noticed, I've used "Cool" a lot. And it's because, to me, all of the new characters, the plot-related characters, only seem like "Cool" tropes, and not like Actual Characters. They aren't characters, they have no personality, they are Assassins. They kill. They kill good. That's about it. And I don't know about you, but I don't think that makes for a strong cast, for something that could keep itself afloat for very long.
Maybe I'm just a character snob.
Most of my complaint lies strictly in the push, the rush, to try and become something that it isn't. To become the next Bleach, Naruto, BNHA... But that isn't what Sakamoto Days is. Sakamoto Days is fun. It's light-hearted and easy going. It's a comedy with wonderfully done action scenes sprinkled in for enhanced gags, or to give character depth and growth.
Sakamoto Days Is Not Meant To Be A Big Name Shounen.
And whoever is pushing it to that needs to stop, to take a step back, and evaluate what makes Sakamoto Days good, and what will keep readers' interest. Especially older readers who are more likely to get bored if a character, if a story, are only "Cool" and not "Interesting". Because guess what! Cool and Interesting are not interchangeable, they can certainly coexist, and often thrive together, but I'd personally take "Interesting" over "Cool" any day. Interesting ahs substance, something to really sink ones teeth into, but Cool.... Doesn't. It's all aesthetic. And Sakamoto Days shouldn't be All Aesthetic with it's artstyle and lack of extended enjoyable cast.
But... Yeah. Y e a h. I repeat myself a lot and have a weird train of thought, so sorry if this is generally incomprehensible or an absolute mess.
Overall; I want Sakamoto Days to be good, I want it to do well, but I'm afraid for it's longevity with it's weak plot, and weak extended cast weighing it down.