The Downside is indeed you need to add the I/O Yourself. But for this it was ideal as I wanted one cable going into the box and that would be it. By having the ethernet phy broken out it meant I could wire the RX & TX Lines for 100Mbps ethernet to the NanoPi and have the Brown and Blue pairs used for + & - Ethernet. I didn’t break out any of the USB ports so there’s no option to add any USB Devices. But the NanoPi Duo has built in wi-fi as well (however reports are it isn’t great, but I was more concerned about fast ethernet)
I was considering using a Raspberry Pi however this would have required POE Splitters or such, While the 3B+ has now come out it’s not very clear if you can do a passive POE implementation (And the cost of the Pi is more than the entire backplane including the nanopi duo).
As you said the size is rather nice. The board is 8x7.5CM If I remember correct and fits reasonably well into a 10x10cm box.