Since I don't work for AWS I'm allowed to say that at the scale of millions/billions of microVMs you're better off running them on bare metal instances to avoid the overhead of nested virtualization.
If I remember correctly, Firecracker VMs don’t have the same security guarantees as EC2 instances. I think I remember that AWS doesn’t put multiple accounts lambdas either on the same bare metal server or VM. I can’t remember which
