That still puts you a single firewall mistake away from disaster. It also places a lot of trust into the applications and hosts that can access ES on a network

That still puts you a single firewall mistake away from disaster. It also places a lot of trust into the applications and hosts that can access ES on a network level: They get full access with no control at all. To add on that: No security also means no TLS, neither in the cluster communication, no TLS speaking to the client etc.