The full explanation deserves a blog post, but in a nutshell it revolves around the issue that ES contains a huge amount of complexity around a feature that is actually fairly useless (the "elastic" part) or at least difficult to use correctly. I've found that you need to be a deep expert in ES to architect and run it properly (or have access to such expertise) and even then it requires regular care and feeding to maintain uptime. In a short-deadline startup world you probably won't have time for any of that--once it's working it will lull you into a false sense of security and then completely blow up a few weeks/months later.