For me shounen is some sort of light hearted novel to read without giving it too much attention. Basically the only shounen I still read sometimes is OP only because nostalgia.
It is targeted to teenagers, young adults. I'm a young adult but still this is stupid since:
1) You know the general line of the plot even before starting the manga: the young hero is an underdog but through some fortuitous encounter he actually gets the chance to become big and then it is an arc after another where he/she overwhelms increasingly impossible odds until he eventually defeats the big final villain and saves the world. So the genre is already wicked in a way since you can predict the plot to a large degree, for who says that every story is like this then try a game of thrones and a ton of non shounen stuff which of course is way less predictable.
2) While when you are younger maybe you can even enjoy the "hero vs baddies" trope that is heavily pressed each arc with time that becomes irritating. The world is not black and white and indeed good novels use various shades of gray with results on a whole other level.
3) With time usually the MC suffers the Jesus sindrome and becomes painful to watch, sadly it happened to OP too even if Luffy managed to resist for decades, now we have Nika and all the hope is lost for OP even in this regard.
4) This is not really a point since it is a bit part of the points above and is not a characteristic that defines shounens but isekais instead and that is "wish fulfillment". Edgy, arrogant and stupid villains get defeated by an even more edgy and arrogant MC, all the secondary characters, even the helpers of the protagonist, are empty shells just put there to give even more light to the MC. But as far as I know shounens usually don't get to this level of bad (they die first luckily).