A unikernel is a single-process system that is specifically designed to run only one application. It is a great fit for modern cloud environments because of its performance, security, and size.
Linux systems have archaic design concepts rooted in the 1960s from when machines cost half a million dollars and needed to run multiple programs for multiple users. Today, developers go out of their way to isolate programs from each other if for no other reason than manageability concerns.
A unikernel embraces the single-process concept while allowing the use of multiple threads. For languages such as Go, this fits well. For interpreted languages such as Ruby and Python, developers in these languages typically load balance a set of application servers to enable a greater degree of concurrency. In the unikernel world, we do the same thing but those app servers become full-fledged VMs and can make use of existing load balancers without having to do backflips.
This is a security design constraint. There is no shell to remotely log into and there is no concept of users. While OPS has a stubbed/fake user it is only present to implement underlying libc calls and has no relevance otherwise. This also means that modern Unix permissions don't have much meaning inside of a unikernel because there is only one program running and no users.