My two main beefs with modern Shounen are
1. Too dark. Every character is a Mayuri or Orochimaru, there are no high or light moments which make the darker moments seem impactful, and characters, plot and world building become an afterthought. It's like manga are either tryhard seinen right off the bat or they're super saccharine romcoms with nothing in the middle. Even an extremely dark series like HxH or AoT had lighter moments and explored characters for more than a chapter before binning them unlike JJK.
And 2. Less world building. The universes of JJK KnY and MHA (and from what I've read of KGB) and the stories within them just seem miniscule compared to older Shounen, and the sense of mystery about those worlds, their factions and their history is next to nonexistent.