Apart from the compliance and audit components, are there any technical issues in installing Issuing CA (Subordinate CA) on a VM. I am finding it hard to convince my client not to go down the path of virtualization of CA servers.
no there are no technical issues. However you should follow virtualization host physical security. In addition you may need to configure your hypervisor to shutdown guest VM when virtualization host is restarted or turned off (in Hyper-V the default
behavior is to save VM state during this process).