That the value of the land might go down? Risk means financial downside, you can lose money on real estate on a multitude of things, millions of investors do every year.

Real estate is not as easy to diversify as stocks and bonds, certain unexpected and uninsurable external shocks can have disastrous effects on the profitability of your investment. Things such as new rent control, property tax increase, termite damage, spiteful tenants, corrupt HoA, and sudden market volatility can all cause you to lose significant money.

I'm not getting the message behind the Churchill quote, landlords clearly have operating expenses to commit towards keeping a plot of land and shelter in useful or livable condition.