Long term real estate investors should not worry about the risk too much as long as you don't over extend your investment (too much leverage). The landlords should not be too concerned about the value of the properties since these are just paper gain or loss. The most important thing for the landlords are cash flows. As long as you have positive cash flow and you can service your loans. You don't have too much to worry about.
During 2008, the real estate market went down a lot around the country and the landlords might see their rental properties value went down. But the rental market went up since there were more people who can not afford the houses or lost their houses due to the downturn. It won't be a bad idea for the landlords to go through a stress test by using 2008/2009 scenario to check whether their investments are too leveraged.