I think student debt is the safest of all. It can't be discharged in bankruptcy anymore. Further, it's typically guaranteed by the government on top of that. Credit cards are a poor comparison I think.
a private student loan is not a particularly safe debt to issue. even though the loan cannot be discharged, there is still a substantial risk that the borrower will never be able to pay it back. credit cards and personal loans are a pretty good comparison IMO, as the main difference is the fact that student loans cannot be discharged. this is in fact priced in already; even private student loans have much lower interest rates than any other kind of unsecured loan.