Once you have set up the namespaces you drop all capabilities so if the program gets hacked while it's running it can do very little.
They can do very little anyway, that way.
Styrolite is a container runtime engine that runs containers in a virtual machine guest environment with no shared kernel state. It uses a type 1 hypervisor to fully isolate a running container from the node and other containers. It's similar to Firecracker or Kata containers, but doesn't require bare metal instances (runs on standard EC2, etc) and utilizes paravirtualization.