Hmmm - fixed income - whether federal bonds including treasury or agency, municipal bonds, or CDs. And some would argue annuities are assured…or 99.9% assured statistically speaking.
That’s the point - with property you are taking a risk. You might do well, you might have appreciation but…you might have expenses or worse - no renters.
With fixed income sources from government bonds or CDs, your risk is inflation is higher and your cash stream can’t keep up - but youR income stream is virtually secured or risk free. The feds don’t default nor does it agencies. Nor do investment grade munis. And nor do CDs which are insured (up to a certain level per bank).