The physical assembly isn't the problem (although you can find plenty of stories of homemade printers catching fire from an electrical short or a thermistor problem that led to unlimited heating of the extruder until the filament caught fire.). The problems are:

• getting the bed so it's in the same plane as the X motion and Y motion,
• getting the rate that filament is forced into the extruder correct for the rate at which the extruder is moving in x and y, (and how fast should the extruder move)
• giving the extruded layer enough time to cool between layers
• finding the correct temperature for the extruder and the heated bed
• the right cooling from the fan blowing on the finished material
• getting x,y,and z rails at exactly 90°,
• calibrating the number of steps in the x,y,z & filament drive steppers so that when you ask for a 1" cube, you get one that is in fact a 1" cube, not slightly bigger, smaller, or skewed in one axis or another.
• Getting the retraction at the right speed and amount so you don't get stringing as the head extruder move from one high point to another.