For a GM the difficulty of an RPG comes from two primary factors. The first is how difficult the rules are to use and the second is how hard it is to get content to run for the game. Some RPGs provide content in the form of Adventure Paths or published sandbox campaigns. Worlds Without Number attempts to guide the GM into how to create content themselves and will be useful for any GM running a fantasy campaign. Even published adventures expect a certain level of improvisulization and content creation from the GM.

WWN also comes with a complete RPG system. The author has been making and remaking this system since 2010. It is a d20 OSR inspired combat system with a Traveler 2d6 skill system. Characters are defined by their Background, Class, and Foci (feats). Characters are human or human like by default, if their race/species has mechanical significants they can take a foci at level 1 to represent that. There are 22 foci race options. This game is more about the decisions before and setting up a combat than what specific things you do in combat. The rules reflect that. While combat is thin on tactical depth (and therefore runs fast), there is rules for major projects, crafting items, and magical workings along with exploration rules and many foci are specifically about social interation. This is not the kind of RPG where you spend 1-3 hours in a combat each session.

The Deluxe edition increases your class combinations to 81 and includes rules for increasing the baseline power level of your campaign. Heroic characters are more like action heroes than regular skilled people. They take on greater odds with less risk. Legates are high fantasy mythic characters. The Legate rules can be used at the beginning of a campaign or as epic level progression for Heroic or Regular campaigns.

This is one of the best RPGs.