virtualbox.org

With a Windows host and guest, the host connected to a secure private network and the guest connected to a different network with internet access. Only the vbox protocol is binded to the network card in use by the guest but usb devices etc are shared. There is not network connectivity between the two.

How at risk is the hosts network from an attack via the guest or is it safe to say it is completely sandboxed? Is it safe to do this

Are you able to reach the host's subnet from the guest? If no, then no risk. If yes, the same level of risk with 2 physical machines.
Your question is only a network security one, not a Virtualbox security question