Its funny how in Japan, people who write stories like this can get lucrative licensing deals while in most other countries, they would be laughed at.
These series all have one thing in common : The MC is obviously the author's self insert and the people he takes revenge on are obviously the author's bullies.
You know what makes it so obvious? The sheer amount of hate written into the story. The way the villains are ALL over the top, ridiculously evil, they always betray the MC for some ridiculous reason like "black hair" or "hes from Japan". There are no shades of gray here, the MC is always a perfect 10/10 good guy who was unjustly betrayed by people who are worse than Hitler.
I mean, look at this series. The MC was a hero that defeated the demon lord, the kingdom's strongest warrior, and he was betrayed by a kingdom spanning conspiracy because he turned down the princess's hand in marriage. Replace a few words and suddenly this turns into the stereotypical "nerd revenge fantasy" in the west where the nerd who was helping the hottest girl in school with homework was betrayed because he decided to stop doing her homework. Then of course he gets revenge by getting a 100k a year job after graduation while the girl becomes a single mother or something.
Apparently in this setting, advisors dont exist, nobody pointed out the sheer stupidity of this conspiracy and the princess somehow has total control of a kingdom (in history, kings often struggled to exercise power because pissing your vassals off meant they would rebel). Where is the king or queen even? If shes already the monarch, she should be called the queen. The daughter of the monarch is only called a princess if she has not inherited a title.
So after the MC dies to this kingdom spanning conspiracy, which resulted in everyone he cared about being brutally murdered in horrible ways (you know, just to make his revenge justified), he resurrects by...GET THIS....convincing a goddess to resurrect him with OP powers so that he can kill everyone in horrible ways. And he pulls this off because the goddess is so madly in love with him that she is practically dripping wet trying to stop herself from tearing his clothes off.
The entire premise is so ludicrous that you can't take it seriously. If you were to describe the premise to your friends, they would probably burst out laughing at how silly it is. Its like some low budget movie that gets aired on friday night about "zombie sharks" or something stupid, and you watch it while drunk with your friends and laughing at how stupid it is.
Except that this is not a comedy series nor is it going for the comedy effect. Its "supposed" to be taken seriously. And thats when you start thinking "what the hell is wrong with the author?".
I mean sure, you want the MC to be betrayed. You want him to take revenge. OK. So you write a plausible story. This is not hard. For example, after defeating the demon lord, the MC gets drunk on power so the kingdom has him assassinated. Or maybe a foreign power had him framed for treason. Lots of plausible possibilities there.
You dont write a ridiculous story where the MC is a perfect 10/10 good guy who did nothing wrong then gets betrayed by a kingdom spanning conspiracy for some ridiculous reason. It just becomes impossible to take seriously, especially when all the villains are over the top evil instead of ordinary people who were just doing their job or who thought they were doing the right thing.
Example : The MC kills a certain character. Instead of making him a normal guy who was doing his job or was tricked to believe the MC posed a threat, the author makes him a crazy cannibal that slaughtered the MC's village when he was a child. Because you know, gotta make them EVIL. He even made sure to draw the character with a crazy "i am so evil!" face. Its like dead pool kicking a puppy, except unironically.
Worse of all, it doesnt even do revenge porn well. I dont feel any sense of vindication when the MC takes revenge. When Guts and the band of the hawk actually managed to bring down an apostle during the golden age arc, it felt like "yea that asshole had it coming, thats payback for all the people you killed, good job guys".
When the MC takes revenge on these guys who only showed up for a few chapters for the sole purpose of being brutally murderered by the author's self insert, the reader has no connection there. Its like watching a zombie movie where new characters are introduced then killed 10 minutes later. You dont feel any pity or sense of empathy because theres no connection.
There is no plot. There is no character development. There is no tension because the MC is so OP that he encounters zero difficulty and he doesnt need to carefully plan out his revenge like Count of Monte Cristo, he can just use his OP powers to brute force any outcome he desires. The revenge scenes arent even good because you dont care about these characters that were just introduced for like 3 chapters. The characters are all one dimensional, "i brag about how evil i am" and "i shit my pants while begging for mercy" are about as complex as they get.