Wrong. Cracking the app does not give you access to the VM host. These apps are running in a VM. There's no relationship at all between cracking the app and cracking VM host.
App exploit (-> LSM breakout?) -> local to root escalation -> VM to host escalation via device.
For a unikernel deployment that's just:
App exploit -> VM to host via device.
It does move the hypervisor up a few levels of abstraction, which could be dangerous, but (more to the point) the benefit is isolation from other misbehaving apps.