Well that is scientifically a half truth, as there will be no downside in an indexed annuity, and that part is the full truth.
However, the upside is always limited by either a cap, or an annual fee, or a participation rate (as in the 80% of the S&P example above).
Therefore, you should know this in advance.
If it sounds too good to be true, then of course you are likely hearing a half truth. After all, the insurance company is living in the same world as the stock market. It would be impossible for an insurer to offer 100% of the markets growth and protect principal 100% of the time as well.
Instead, they must manage risk by limiting the growth in the upside for those times when the market does fall. In all of the above strategies, the policy owner will never see a decline in their accumulated principal.
In bad times, the worst an owner can earn is nothing. In good times, there are some serious gains to be had.