So i have a first gen Sonoff POW (not the R2) that monitors the consumption of my Home Assistant PC, cameras, wifi access points etc. It is probably 3 years old and pretty much running 24x7. Not really high current - max like 500W. I did use it a while back with a 1500w oil heater.
I noticed tonight that there was some arcing occurring from the unit and immediately shut down everything and plugged the Sonoff out.
Turns out the first gen with the fuse clip has a problem where the fuse clip itself gets hot, conducts the heat back into the board and eventually fries it. Lucky I caught is early.
It was also the last one where I did the cut-up-the-lead-put-Sonoff-inline trick - definitely not recommended!