I am often wrong about such things, but I'd say the lesson of the hunt for app tech unicorns over the last few years is that they are trendy, become super popular overnight, then have a hard time finding an actual revenue model, fail to somehow eat the entire world a la amazon and facebook in their own ways, and finally see the user bases fade as something else trendy comes along.
Money gets burned away, businesses expand to crazy levels, and then what would probably have been be neato profitable niche products get destroyed (or, in some cases, bought by one of the big players and then destroyed, though that's a slightly different story).
