These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Crash Course
Artistic License-History: John Green skips over the Protestant Reformation in World History and the Space Race in US History. The closest we get to the latter? An image in the introduction briefly mentioning the Apollo Program and a still image of an Astronaut in the episode on America in the 1960s. And what does Green make a video of instead? Captain Cook. Is Cook an important figure? Debatable, but it is certain that he isn't as important as the Reformation.
Broken Base: The episode "Battles of the Civil War" has proven to be this. Some viewers accepted John's basic listing of major battles of the Civil War as a good Take That, Audience! towards those who ask for more military history on the show and acceptable filler to pad out his paternity leave, especially since the following two episodes were also about the Civil War and contained much more analysis and details. Others dislike the episode, either because they see it as disrespectfully trivializing the entire field of military history or because an intentionally boring show is still boring.
Fan Dumb: John goes out of his way to point out how historical bias and Eurocentrism has altered how many people view history and therefore other cultures, with episodes dedicated to cultures and eras that have either been ignored or misunderstood mostly because they didn't affect Europe. Many fans have appreciated learning about these new peoples ... except for the few who think that the show is 'anti-European' and 'unfair' and 'always criticizes Europe'. Because trying to counter centuries of historical bias with some friendly ribbing is just being mean.
Some manage to come to the opposite conclusion, a Western bias and ignorance of Eastern culture and history. He just can't win.
Green admittedly does go out of his way to criticize Europe, to the point of extreme overcompensation for previous Eurocentrism.
Crashcourse has also covered world religions, but only in a factual historical context that examines that religion's effect on culture and history, not if that religion is actually true or not (as John says, this is history, not theology). Still doesn't stop endless Youtube comments fighting over religion.
It speaks to how divisive the issue of America is that the comments on the US History videos constantly accuse him of being both pro-American and anti-American. The ones accusing him of being too pro-American are often Sarcasm-Blind enough to not realize that the "Libertage" is a tongue-in-cheek mockery of American patriotism. There's also all the fun Misplaced Nationalism fights, especially between Americans and Canadians over who won the War of 1812 and that started several videos before John even covered that war.
Predictably, John's video on the George W. Bush administration has attracted all the usual criticism from the 9/11 truther crowd.