Watch BigClive on YouTube and his adventures in reverse engineering to see the very mixed bag of cheap modular PCBs. Some of the USB chargers scare the hell out of me!
The quality of pre-made PCB modules varies dramatically from “can be made to work” to “likely to explode”.
Some modules are reference designs from a datasheet, some are copies of copies of copies - the latter can be missing rather important parts as the final designer had no idea of the original circuit.
My most hated example was a 12V 4x relay board - ESP12F looked good, but some moron copied the design with an I/O expander chip, and didn’t bother to program the I/O expander creating £10 eWaste that never ever could work (no, not the ESP firmware - the unobtainable ST uP used as an expander to save $0.01).
I’ve bought many cheap PCBs as they can be easier than making something yourself (240V mains example - made safer with battery isolation), but consider the time needed to get them working - the only available datasheet might be from a design three copies ago, so doesn’t line up and you waste half a day.
So, for a quick test rig, with additional safety precautions - proceed with caution.
For a device which is hard-wired and running 24x7 - save yourself the heartache and buy a Shelly EM!
It’s not just the risk, it’s the time you’ll waste.
(And I say that just after replacing a Tuya CB3S with an ESP8266MOD on an IR Blaster and spending half a day messing about with strapping pin 10k resistors to get the new micro to boot reliably. Sometimes it’s fun; sometimes it isn’t!)
If this helps, 
this post!