We don't think monolithic firewall-based security has been very successful at preventing hacks. Our goal is to create an environment that involves much more fine-grained separations, and enforces security properties at the platform level so that bugs in apps are largely mitigated. We want you to be able to deploy apps without having to security-review them first, which means the platform itself must provide guarantees.
Arguably an app-driven SSRF is a pretty big problem in that threat model. I think we missed it earlier because we imagine a future world where people don't expose unauthenticated services on the internal network and rely on their firewall to protect them. Of course, we need to keep in mind that the existing world isn't going to go away when people deploy Sandstorm and so we need to handle both worlds gracefully.
Another point to make is that we do envision use cases where someone sets up a personal server and invites their friends to it to chat and collaborate -- usually as "visitors" (can't install apps), but sometimes as full users sharing one server. Typically you'd only invite trusted friends to be "full users", though, unless you are running a hosting service. Hosting services (like ours) ought to be extra-careful with multiple layers of security.
