There's been multiple real estate crashes in US history...
http://www.businessinsider.com/the-economic-crash-repeated-every-generation-1800-2012-1?op=1
It's not like it was always a rock solid investment up until the last crash. If you were to compare real estate crashes to key issue crashes you would see that the differences are nearly polar opposite.
It's only dangerous If you use the past as an absolute, and aren't prepared for the worse, which is something a lot of real estate investors weren't prepared for. A lot of those people weren't even prepared to buy real estate in the first place, let alone prepared for the worse. Using the past to predict future outcomes can be a great tool when utilized correctly, though it can be dangerous when the full spectrum isn't taken into account. I think people took the real estate market value for granted, and If they had taken a closer look at how real estate has played out in previous times, they may have been a bit more hesitant to invest their money into it. I think ignoring past behavior can be an even more dangerous move than trying to incorporate it to your advantage.
Comics can be relatively inexpensive (especially compared to real estate). If someone was to invest $800 in Hulk 181, and the book crashed, then so what? You may have lost $800, but you certainly won't need to file bankruptcy (unlike the real estate crash). On the flip side, If someone was to invest hundreds of thousands of dollars into the same book, then yeah, it's risky, because If that book crashes, you now have to face a major loss. The solution here is to not put all your eggs in one basket. Regardless, there's always two sides to a coin.
I'm not saying it's impossible for there to be an AF 15 crash, or a Hulk 181 crash, but I am saying that investing in comics doesn't have to be as risky or dangerous as some people here are making it out to be.