A unikernel is running a single process in a single address space. So yes, if you compromise the app you compromise the whole system but the whole system is the app.
You could potentially compromise the rump kernel, but you still wouldn't be able to break out of the VM's isolation context.
On top of that, you didn't get to reap the benefits of any of the protections you get from a proper Kernel like Ring 3 execution, NX bits on pages, guard pages on the stack, etc.