Complex sentence: The great advantage is that only the masses of the peptides have to be known (so no "de novo" sequencing); a disadvantage is that the protein sequence has to present in the database of interest.
Split into simpler sentences: The great advantage is that only the masses of the peptides have to be known (so "de novo" sequencing is not necessary). A disadvantage is that the protein sequence has to be present in the database of interest.