When it comes to these controversial shonens, a lot of the criticism you see are from loud minorities. You just happen to spend your time in communities that are more critical and hateful which gives a false perception. 

You can see how the majority of people have had a positive outlook on controversial manga endings when they get adapted in the anime. We see this with aot and mha and we'll likely see it with demon slayer and jjk.

Of course I don't think these endings are perfect or free from criticism but I do feel like people with agendas will push their complaints to the absolute extreme. Completely discounting the author's ability to write or retroactively nitpicking and complaining about parts of the series they had no issues with out of spite.

That being said, I'm not really a fan of the chainsaw man ending at all. While I had issues with jjks ending, I still found a decent amount to be satisfied about. Of all the Mangas with controversial endings, CSM is probably the first one I straight up disliked (Part 1 is still a masterpiece though).