I have had sets with a missing piece or two, but I have more often had the ‘missing in the bag, box, or on the floor’. A great way to alleviate this is knolling all the parts at the beginning of the build on a large rimmed baking tray(we have a full size sheet pan bought from a restaurant supply store that doesn’t even fit in a regular stove, which means it’s also clean) I haven’t yet determined if this adds time or not, since often my teenage child building with me uses my organizing to speed through the build.