When using any non-metallic wiring in an outdoor application, you should run the wiring through a conduit. This includes situations where you might consider using Romex. Given that exposed Romex needs additional protection to avoid contact with solvents and moisture, it’s clear why it isn’t recommended for outdoor use without proper conduit protection.
Compared to conductors, the copper ground wire inside is regularly exposed to various elements. When running the wiring underground, it will be exposed to weather, water, pressure, and roots. If you are using Romex, especially stripped Romex, the need for a conduit becomes even more important. When Romex comes into contact with solvents, the bare copper ground wire can negatively impact the connection and trip the breaker repeatedly.