> In the past I have worked in defence, for highly sensitive stuff they wouldn't even allow a common ground between two networks.

I actually agree very much with this. If you're looking for strong assurance that there is no possible back-channel, devices like optocouplers help significantly. It's not hard for me to think of a way to surreptitiously send data backwards through a common ground, or normal silicon diode, or a magnetic coupler like an Ethernet transformer, but optoisolators make it significantly more challenging.