I don’t know about that my friend. I’ve only ever bought one Sonoff device. Their fan controller 2. When I got them and looked at the quality of the circuit traces and the cheap relays they used, I had reservations about putting them in my fan canopies. I decided against that. I know you are a bit more savvy than the average consumer here for sure.
I think I would give stronger pause before embedding such a low cost device behind my light switches. If that were to burn up behind the light switches, flames could go straight up your wall. I get that low cost attraction and I actively participate in those type of purchases, but you should maybe think twice about this one. Just some friendly advice.
One thing that manufacturers of low cost devices will skip out on is the flame retardants in the plastic. Just some food for thought. Be safe 
On a side note, that looks like the identical size to the Xiaomi dual relay, which is flame retardant plastic and has good quality relays as you know. Maybe better to spend the extra dollars for such an application.
