Some people point to Obama's statement a couple of years back that Social Security checks might not go out as evidence that the debt is not secure. To which you can only reply... If the president was telling the truth. SSTF debt is better than debt held by the private pensions in that it can be redeemed without affecting the level of debt for the country. So the Trust Fund can tell the government to refinance the debt without respect to the debt ceiling.
The issue of negotiability is brought up by people who seek to deceptively discredit the assets held by the Trust Fund in the minds of people who do not understand how capital markets work. At worst, you can depict the debt as a private placement. That doesn't change the comparison to other obligations from the same issuer.