Already starts the story off where the (Generic) MC is at his peak. No character development, no world building, and thanks to the "Quiet Life" mindset, the story has no end-goal.
The harem is there just to flaunt and flex that the MC has lovers, despite the fact that he has no courage or willpower to commit baby-making.
Also, without rhyme or reason, not even a lengthy explanation on how it was made, or how he acquired the knowledge, the MC has a gun.