The perfect mechanical elements of the gameplay that are in place
means enormous amount of work and time are required to tweak the mechanics for individuals player to provide them with unique, dribbling animations, running styles and shooting style without breaking the outstanding mechanics in place and being able to adapt to the other mechanics such as the collision engine.
In the abstract, I would say that the most difficult part
of game development, especially for a self - funded independent project, is finding the
means and / or time to
work on the project, dedicating years
of your life and an
enormous amount of intellectual, physical, and emotional energy into a project that could ultimately fail completely.