It's very hard to do this kind of thing by slurping logs (and is generally why ELK isn't considered a replacement for these tools). The first thing applications like Haystack try to solve is context. While I can't speak for Haystack (as I've never used it), Sentry using it's SDKs pulls in a ton of information that you would never get, or would at least be very hard to pull from logs. Examples of this include most of the HTTP request information, the authenticated user, context locals and surrounding code in the stacktrace, as well as any arbitrary context you might want to associate with it.