I'd say it contrasts greatly. But the only reason the American melting pot system is more successful is actually in part due to the type of background it receives it's immigrants from.
The number one minority in America are Hispanics. Either transplants from white colonies or from Mexico/Latin America. They are Christian, speak a European language, and in most cases can learn English well enough to function in society. Partying is very popular with their youth, as is with American youth culture as well. African-Americans in second place have no language of their own, and their culture/fashion sense is simply a slightly twerked version of North American culture/fashion. These two groups have little to provide in terms of cultural opposition to the States. Asian-Americans for the most part make up less than 10% of the overall population and are overwhelmingly concentrated in the larger cities where the diversity is more manageable for them.
On the other hand, Canada predominantly receives immigrants from Asian cultures. These cultures tend to be far more conservative compared to Hispanic/Latino and African-American culture seen down south of the border. Especially cultures from South Asia and the Middle East, which are notorious for directly opposing much of Western ideals when it comes to freedom of expression, etc. This creates a direct divide between the Western culture and the Eastern culture which is tearing into the individual's need for recognition. Because of this, assimilation is much less likely, bringing up the need for questioning the role of "multiculturalism" when the result is a large portion of immigrants refusing to speak the local language and follow a religion with vastly different ideals.
Not to say several American cities don't have their fair share of "disastrous" multiculturalism themselves, but it's more of a prevalent notice in Canada due to the migrant pool it draws its immigrants from. If Canada started letting in Latin American immigrants as the dominant ethnic group, there would be a lot less debate on the morality of drinking alcohol or dressing provocatively in front of men. It's all relative.