That line is the biggest BS I've ever heard. To an extent, you sink or swim at both US and Canadian Unis. No professor/TA is going to come after you if you are failing. They are equally hard.
In Canada, all of your classes will be HUGE (50-400) people until 4th year. With some exceptional cases(like aerospace eng), there will be 10-20 people. In the US, classes can range anywhere from 10-400 but it is way more common to have a small class. I go to a Public State Uni with 25k people and out of the 15 classes I have had, 8 of them had 20 or less people.
I have friends at both US and Canada universities. They are both challenging. In the US, you will have a more holistic education (you will sample everything since you have general education req.) whereas in Canada, you will stick mostly to your major.
Its much harder to get A's in Canada whereas the average grade in the US given out is a B+/A-.
In the US, you will get a much more traditional college lifestyle whereas in Canada, it is much more of you go to school to get an education and you get out, not a place to figure yourself out.
They are vastly different. It would help if you would provide more info like Liberal Art Uni, big public uni or private university, since there are so many types of schools in the US.