In small quantities, immigrants provided needed influx of ideas and perspectives. In excess, there are too many to safely assimilate, and instead immigrant groups form distinct minority groups, which can lead to political factionalization. It's not that bad in America, where enough wealth is located in the hands of the Caucasian population to keep them in control regardless of demographics, but it could lead to long-term problems when everyone else gets upset with the dominance of rich white people. Even if immigrants are of the same ethnic and cultural background, as Amos suggested, simply dealing with large numbers of people with little capital, especially in an already poor market, can strain economic capabilities.