I'll feel pessimistic like this, but then something like Tinfoil Chat [0] comes along and sparks my interest again. It's still all just theoretical to me, but at least I don't feel so bad about things.

With a little bit of hardware you could get a lot of assurance back: "Optical repeater inside the optocouplers of the data diode enforce direction of data transmission with the fundamental laws of physics."

[0] https://github.com/maqp/tfc