I suggest that everyone read the Social Security Law. Surprising what you'll learn. I was an administrator with the Social Security Administration for 32 years and have, at least, some knowledge of the law and the funding of the program.
Supplemental Security Income is a program funded out of General Tax Revenues and is a financial needs based program. In layman's terms, it's welfare. The income and resource limits are very low. To be considered "Disabled" you have to meet the same requirements as those for Social Security Disability. The program is administered by the Social Security Administration and ALL payments and administrative costs come from General Tax Revenues, NOT from Social Security Program income.
The Social Security Program is financed through FICA and Self-employment Taxes. It is a "pay-as-you-go" program. That is, when you were working your FICA or Self-employment tax paid for the benefits of the then retirees. Todays workers are paying for your monthly Social Security benefits. Any program income not immediately needed to pay monthly benefits and administrative costs (which are less than 3% of income) are invested in U.S. Government Securities at current interest rates, the same as you or I invest in Government securities. Is it an IOU? You could say that. But isn't your CD at the bank an IOU from the bank, or your checking or savings accounts, etc., etc? What does the bank do with your money on deposit there? It doesn't sit in a container with your name on it, but is invested by the bank in home mortgages, property, etc., so you can be paid interest on your CD, etc. Which would you rather have, an IOU for billions of $ from a bank, that could default, or the same IOU from the U.S. Government? The Social Security Law states that if the Social Security Program has insufficient funds to pay current benefits, those benefits WILL be paid from the General Treasury of the U.S.
Will the Social Security Program go broke? NO!! Despite what many politicians want you to believe, Congress will take whatever action is necessary to keep the Social Security Program solvent by increasing income to the Program. This can be done in any number of ways, too numerous to go into. Every politician and all political party's know that with over 25% of the population receiving Social Security Payments each month (retirement, survivors and disability benefits), and millions of voters knowing their parents, grand and great-grand parents are counting on the Social Security Payments each month, it would be the political kiss of death to have the program fail.