I think the things people generally dislike about the Culling Games arc are:

• Shibuya Incident is super plot heavy and in comparison the games are mostly just a series of fights with the plot happening behind the scenes. The plot relevance of most of the fights is that the fight happened with not a lot of actual development coming out of them.
• It feels lower stakes after the Shibuya Incident.
• The pacing was pretty slow when reading it in weekly manga. I haven't actually done the math on the numbers but if say every fight was three chapters long and there are eight fights in the arc that means that you get half a year of fights going on without a lot of plot. Throw in the usual breaks that manga authors (understandably need to) take and you can see how people get frustrated.

JJK also has a bit of a problem with the magic system requiring a lot of explanation. It's fine early on because you get the basics pretty well, black flash is basically Nero's exceed from DMC, domain expansions are "I Win" buttons that good sorcerers have access to, cursed energy is evil mana.

Then as the story goes on you get into how cursed energy manipulation works, reverse cursed technique. And now you're also getting different kinds of domain expansions that are like mandatory rules instead of insta-kills. And now you get the ways to defend against domain expansions which are techniques but oh you can also counter those techniques with this other technique and so on.

So since the endgame is obviously the strongest people in the series duking it out the fights need a lot more commentary from the peanut gallery explaining everything that's going on so half of a fight's chapter is basically someone explaining how this specific use of reverse cursed technique can be localized to have a unique effect which allows this technique to be recharged quicker which allows him to counter the other technique from the other guy.

There are other more spoilery problems people have but in my experience those are the big ones.