If you plan on investing now and letting the money sit there for the next few decades, stocks are the way to go. The occasional slump won't hurt much in the long run.

If you plan on withdrawing money every year for living expenses, then things get tricky. Taking out a fixed amount each year will amplify the effects of stock market slumps. You might get low enough that you're withdrawing all your returns for the year. That leaves you running in place, falling behind inflation, and one recession away from getting completely wiped out.

The risk here is the risk of ruin. Once your investment hits $0, you're out of the game.