The article does compare them (albeit without much detail).
“Delinquency rates on student loans are much higher than those on auto loans or mortgages, due to loose student loan underwriting standards, the unsecured nature of student debt, and the inability to charge off non-performing student loans in bankruptcy,” Goldman Sachs Group Inc. analysts Marty Young and Lotfi Karoui wrote in a note Tuesday. “The substantial majority of student loan default risk is borne by the U.S. Treasury.”
The risk that the increased supply of money lowers the value of that money. Not that this is the straw that's gonna break the camels back, but it's not like there aren't consequences to the taxpayer for allowing the government to allocate funds in a sub optimal manner.
This money isn't unbounded; it's confined by the budget allocated to the Department of Education and various government organizations (like Fannie Mae, et. al.).
Ultimately the Department of Education is responsible for this money. The last 30 years are remarkable in the amount of controversy surrounding student loans and the default of large education funds.
I think the main risk would be a deterioration in the government's finances since the government is expecting to get paid back and is factoring in those payments to the financial outlook. That could in turn lead to an increase in the government's borrowing costs, which could affect lots of other things negatively.
“Delinquency rates on student loans are much higher than those on auto loans or mortgages, due to loose student loan underwriting standards, the unsecured nature of student debt, and the inability to charge off non-performing student loans in bankruptcy,” Goldman Sachs Group Inc. analysts Marty Young and Lotfi Karoui wrote in a note Tuesday. “The substantial majority of student loan default risk is borne by the U.S. Treasury.”