FiveM is safe to play on, you can even switch between FiveM and vanilla GTAO and you’ll be fine. The advantage of steam version is higher security (someone would need to get inside two accounts instead of one) and for some people it’s just good to have their while library in one place.
Do I need GTA V for FiveM?
FiveM requires a fully updated installation of GTA V. If you do not own a legal copy of GTA V yet, you can buy it from any of the stores below. FiveM does not condone the use of pirated software and will not work with illegitimate versions of GTA V.
Can you use FiveM with Epic Games?
Fun fact: FiveM works perfectly fine with GTA V obtained from the Epic Games Store, including the *free* promotion running right now. Get it if you need it!