Complex sentence: The major advantage to this method over direct addition of the nucleophile to an acid derivative is that it avoids the common problem of over-addition in which two equivalents of the incoming group add to form an alcohol rather than a ketone or aldehyde.
Split into simpler sentences: The major advantage to this method over direct addition of the nucleophile to an acid derivative is that it avoids the common problem of over-addition. Commonly, two equivalents of the incoming group add to form an alcohol rather than a ketone or aldehyde.