Is it the Hiroshima scene? Is that why the crticis scored it lower?
I'm not a big marvel fan, but I think this movie is as good as the other Marvel films.
I'm pretty sure it's the Hiroshima scene. They probably don't like to be reminded that the bombing was a crime against humanity.
If it's about messing with real-world history, I never heard any of them complaining about Hydra helping Nazis.