Do shares always produce good returns over the long run? No. 2 Examples.
There are periods of long-term history when shares do not produce good returns over 20 or 30 years. Therefore it is important that you look for signs that might indicate that the period ahead is one of those periods.
Here are two examples of poor long-term returns from shares.
US shares 1800-1860.

Japanese shares 1989-2009.

One of the indicators of very long-term poor returns ahead, is if that share market has historically extreme cyclically-adjusted P/E ratios - like the US share market (S&P 500) has now.