I am new to the game-dev area, although I have professional experience in programming and some other qualities like making graphics in Photoshop or 3D modelling. Currently I am learning one widely used game engine connected with another free 3D modeling tool and my problem is that from my beginner's perspective, it all seems relatively easy and I know that this is not true. Right now It seems I can find all knowledge I need in some online tutorials, courses and if something is missing I can still ask a question on websites such as this one.
I plan to do an FPS multiplayer game as my first game and I wonder what is the hardest (most time, energy or money consuming) stage of game development in 2020?
I am asking because right now I think I can make 30-50% (prototype and some assets like 3d models) of the game myself in around year period of time and maybe later get more money to finish the project, but I know it is not true and this is only my beginner's unexperienced imagination, because similar games are usually being done by a team of experienced developers and their games still have bugs, glitches and other issues and I know some other similar but amateur projects that were started by one or two people and eventually were abandoned due to unknown reason (mostly lack of time I assume). So I am wondering what are the stages that cause most amateurs to fail and abandon their ideas, is it modeling, programing, networking, debugging?