Once a security has entered bubble territory, by definition its price has become disassociated from any underlying fundamentals - bubbles form when rising price

Once a security has entered bubble territory, by definition its price has become disassociated from any underlying fundamentals - bubbles form when rising prices drive positive sentiment which pulls in fresh buyers who buy into the rising price story, driving the price higher which in turn draws in more buyers in a self feeding cycle that only ends when the supply of fresh buyers dries up. Because this is a self feeding cycle, the price can double & redouble from almost any point if the "story" is good enough. So you can be dead right about a given asset being in a bubble, but if you short it the odds are that you're going to get painfully stopped out long before the bubble collapse finally happens. As Keynes said, prices can remain irrational longer than you can remain solvent: The history of investing is littered with individuals who were right, but lost their capital because they were simply too early.