It can make “bad edits” eg off target effects. But in this case there were, as far as is known, none. It’s aided that this was a single nucleotide defect.

They specifically tested for off target edits in the mouse study and found no harmful edits (and very rare off target ones). That plus the specific targeting of the liver cells (no germ line effect expected), makes this a low risk approach and certainly better than doing nothing.