I saw this module as a "battery emergency switch module" for $2 on aliexpress:
which is just a relay energized by the external power supply, and when the external supply is gone, connects the battery to output. despite a relay could switch higher currents than a same priced diode, it is slow and the chances that the circuit resets are high. also, the relay may stay energized with lower voltages than the nominal supply, while the circuit has turned off because of low voltage.
what could be the problems with a simple circuit like this ?
it is cheaper (for some applications at least) and much faster than the relay. it doesn't have the problem of low voltage mentioned above.