The problem with these indicators is that, although they may indicate over- or undervaluation, they tell you nothing about when a mean reversion will happen. As Keynes famously said: "The market can stay irrational longer than you can stay solvent." An indicator that does a pretty good job of signaling the "when" of a recession, and by extension the likely "when" of large market corrections, is yield curve inversion.…
Another way to think about this is that historically equity returns are so consistently high that even with an accurate predictor of returns it’s not worth not being exposed to stocks. Even if you know the next ten years will be in the bottom decile of returns for the S&P 500, you’re still better off than with cash.
"Suppose I offer you a security that will pay $100 two days from today. You can buy as much of it as you like today at $50, or you can wait until tomorrow. Tomorrow, I’ll flip a coin. If it’s heads, I’ll sell you the security at $99. If it’s tails, I’ll sell you the security at $25. What should you do?
Clearly, if you buy the security today, you’ll double your money two days from now. That’s a 100% expected return for each dollar you invest, over that 2-day period. If you wait, you’ll earn nothing on the first day, but you’ll then have two possibilities. If heads, you’ll get just 1% on your invested money. If tails, you’ll get a 300% return, quadrupling your money. With a 50/50 chance at each, your expected return for every dollar you invest is 0.51% + 0.5300% = 150.5%. So waiting adds 50.5% to your expected return over that 2-day period."