I think a unikernel+ProcOS is basically the same as a hypervisor+unikernel, except backwards. The main advantages would be less overhead (one unikernel, instead of N) and greater flexibility (because ProcOS can run on any OS without a VM).
The emphasis on cheap recursion (if it can be achieved) also has benefits like making administrative responsibilities easier to delegate, and preventing various isolation leaks (including scheduling fairness).
If you run them in qemu then this becomes very clear. Even better (or worse?) the strange system call ABI is translated into regular system calls by the qemu process.
The emphasis on cheap recursion (if it can be achieved) also has benefits like making administrative responsibilities easier to delegate, and preventing various isolation leaks (including scheduling fairness).