You mean the editing? The editing depends on fairly complicated processes that originally evolved (to the extent that one can impute function) to repair drastic problems.

The complicated bit comes in knowing what to change. There are some diseases in which the entire disease is caused by a single mutation and changing that one mutation in the germ line would be sufficient to correct for the disease. However, most situations involve complex interactions with many genes in a way that making a single or small simple change would have many side effects, or cause other serious problems.