Pro Built with security in mind
What MirageOS does in simple terms is to provide only the minimal needed for an app to run. It provides only the required network and hardware drivers needed to execute.
Using this approach it minimizes the attack surface for potential hackers since they don't have any shell to log into or any other insecure cruft that other deployment methods, such as full operating systems have.
Con May be too much for regular development work
Letting developers work closer to the metal also makes them lose a lot of the useful abstractions a normal OS has which are not built for no reason. Considering this, it may not be a good idea to always use MirageOS for deploying an application.