Isekai, and by isekai you and I both mean those male aimed, haremish, battleish, isekai, honestly have one flaw.
Everything else is fine if you like the battle harem genre, but there is that one flaw that inherently ruins it.
And the flaw is that there is no weight associated with the power.
In traditional shounen works, being powerful doesn't make you just special, it makes you part of the warrior class. And being part of this class comes with duties and responsibilities.
On the other hand, a lot of works for girls, starting from precure all the way up to older audiences, tell you that being brave will give you power. Just think about how many precure characters got their powers being by putting themselves in danger to save someone else.
So basically you have a relation between duties and power. Men like to believe that being powerful means you must do the right thing, while women prefer the idea that being right makes you powerful.
In the isekai people complain about, both are lacking. The mc is incredibly powerful, but he can do whatever he wants. There is no noblesse oblige holding him down. And this is bad, because only gods and beasts live outside of the city (laws).
Moids don't realize it yet, but they actually despise the idea of a guy getting special powers and hot women just because he died and then being completely free. And I know they do, the vast majority of people complaining about isekai, at least on 4chan, are men. The male brain needs to struggle and suffer before being rewarded and it needs morals and laws to follow after being rewarded.
Isekai as a modern genre is born from men fundamentally misunderstanding what they like in fiction.
