Not exactly, sites like eBay were penalized in Google for the low quality contents created by users (auctions with small and copied texts. All this can be expected, and if you are operating in accordance with certain criteria, the penalties are avoided easily. Following your reasoning after each update there should be mixed in the SERP and is not so, the changes are there but they are not random. Spam sites are penalized and quality sites are rewarded. Google certainly very often makes mistakes in judging the sites but algorithm can recognize sites with real quality.