This is my main issue with pfSense/OpnSense. Even the dedicated hardware appliances are quite power hungry compared to some of the rather capable Linux ARM boxe

This is my main issue with pfSense/OpnSense. Even the dedicated hardware appliances are quite power hungry compared to some of the rather capable Linux ARM boxes out there. For comparison, an 8 Gbps capable pfSense appliance (like the Netgate 4100) requires 40W-50W (max 60W), where the UniFi Dream Machine Pro, also capable of 8Gbps, has a maximum power consumption of 33W, which includes a 3.5" harddrive for UniFi Protect. Mine uses about 18W without the harddrive, and 22W with a WD Red. A difference of 25W over a year at current european electricity prices (€0.5/kWh) means a saving of 219 kWh (€109/year). Considering that electricity has been as high as €1.12/kWh this spring, it could be even higher. As for virtualization, while it's a great learning experience, it's probably more trouble than it's worth. I greatly prefer appliances for network.