It all depends on what you want to do, and what is the nature of your game project. It can go from really very easy to an absolute pain. Anyone that wants a game that invests largely on 3D meshes, and assets visual quality, will find the engine pretty easy. Anyone that wants to make a game that depends on animation-interactions like DarkSoul, like fighting games will also find it easy. Anyone wants to make a game based on simple physics simulation, will also find learning easy. However, any game, including the previous ones, that will for whatever reason ultimately need an understanding of the source code, or editing it, will be a very huge task that will be ofc difficult.
In other words, if your game is within the standard common genres of games in the market, and does not need AAA level optimization, then your task is really smooth and easy.