SIERREUR (IFERROR) replaces any error in a formula with a substitute value, whereas SI.NON.DISP (IFNA) only handles the #N/A error and lets the others show. Targeting #N/A avoids hiding real anomalies, such as a broken reference. Replacing an error with 0 can distort totals.
Example
=SI.NON.DISP(RECHERCHEX(A2;Clients[Id];Clients[Nom]);"Not found")