Just an FYI -
I've just installed my first 5 Sonoff POW's. 3 worked wonderfully and connected. The other 2 connected intermittently, and were often in the "double-blue-flash" state.
Solution: in these 2 circuits, the neutral was actually the live and the live was actually the neutral. When I switch the connections on the POW, the suddenly connected stably!
The symptoms were that the POW was connecting not connecting to the Wifi (single-blue-flash), or if it connected, it wasn't managing to connect to the Sonoff server (double-blue-flash). I don't know how or why or what in the crossed live/neutral caused this.
Completely agree with everything you wrote, which is why this surprised me enough to post it. My guess is that it has something to do with AC making it into the circuit board. There was another phenomenon too ... the multi-led floodlights that I connected were kind of glowing all the time until I switched the live and neutral, so there was definitely some kind of leakage that there should not have been.