One of the things I'm surprised your article doesn't mention is the tax advantages of this type of investment. The depreciation and rehab costs (purchasing distressed properties) can be huge deductions to ones income taxes, which none of the others have. Then, along with the appreciation of real estate, this passive income investment outperforms the notion of maxing out my 401k as well.
5. Make sure you are properly diversified. Capital preservation is underrated. We saw a lost decade for tech stocks between 2000 and 2010 after the first dot-com bubble burst. It actually took 13 years for Nasdaq investors to get back to even. Investors in the Borsa Istanbul stock market index just gave up 10 years' worth of gains after they saw a plunge in their currency, partially due to increased tariffs by the US and a lack of confidence in the government. Your passive income needs to be properly diversified in order to take the hits.
Residual income is calculated as net income less a charge for the cost of capital. The charge is known as the equity charge and is calculated as the value of equity capital multiplied by the cost of equity or the required rate of return on equity. Given the opportunity cost of equity, a company can have positive net income but negative residual income.