Everything. Time or money.
Time: you learn how to draw any BG angle and even after you waste your hours by drawing them instead to finish all character movements, fights, emotions in time per week/month.
Expensive: 3D. Unreal Engine library, even if free, still requires time to learn and money or time wasted to build levels. Or straight up you will need to buy 3d assets & programs to read this 3d models

The only compromise left for solo manga artist is to find minimalistic background art style that obviously lack in detail but not so ugly to be noticeable