My very reliable Zigbee network on a C-7 started going offline immediately upon adding one of those:
If you read the messages in that thread, I had no end of problems with the device on my main C-7. Sonoff released new firmware (1.0.5) supposedly to help, but updating to the new firmware did not stop the Zigbee crashes.
In the end, I took it off my main C-7 and added it to my other C-7, which had no Zigbee devices. I then used Hub Mesh to being the device to my main C-7. That worked flawlessly.
I have 3 total on my 2nd C-7 and they work just fine that way.