Seems like the foreigner would be more motivated, more likely to do whatever the advisor asks with no complaints, less likely to complain about the poor working conditions and pay of a grad student, and much less likely to say "screw this" and go to work for anybody.com (an option not available for the non-green-card-holding foreigner).
But, more likely to be pressured by their family into a "prestigious" career that they aren't really interested in, more likely to have the additional pressure of supporting a wife and be thinking about kids, more likely to be unable to learn how to write research papers well in English (this is surprisingly hard)...
Also, I've definitely seen people from abroad use American grad school as a temporary stepping stone to get a job at a big company (like Microsoft) that will deal with the visa stuff. Not saying they were planning it all along, but it happens.
Of all the above, I think the "being able to write polished, professional, technical research papers" is by far the biggest risk factor that neutralizes the benefits of foreign students that you mentioned.
But, more likely to be pressured by their family into a "prestigious" career that they aren't really interested in, more likely to have the additional pressure of supporting a wife and be thinking about kids, more likely to be unable to learn how to write research papers well in English (this is surprisingly hard)...
Also, I've definitely seen people from abroad use American grad school as a temporary stepping stone to get a job at a big company (like Microsoft) that will deal with the visa stuff. Not saying they were planning it all along, but it happens.
Of all the above, I think the "being able to write polished, professional, technical research papers" is by far the biggest risk factor that neutralizes the benefits of foreign students that you mentioned.