The thing is the US system is much more at the mercy of individual teachers (the overall grade is determined by the teacher, and only certain aspects are externally moderated/examined - so if your science teacher is a creationist and you believe in evolution, you will be getting failing grades consistently, particularly with the lack of national curriculum) and the long running requirements much more heavily penalises those with long term or chronic illnesses, and those with permanent disabilities, as well as those with mental health issues even still, in a different.
Moreover, the US system promotes learning to take a specific type of exam and doesn't heavily support subject knowledge acquisition. While it's "easier" in some respects and has less focused periods of stress, it is without question a considerably weaker education for the individual - which is reflected in the aggregated perspectives of the population, a significant minority of whom don't believe in evolution, and until very recently an outright majority did not believe in anthropogenic climate change.
This is speaking as an American, who has primarily grown up in the UK with a few exceptions of living in the US, who has mental health problems that have affected my education.