When using any published compilation, abstract, or finding aid, read the preface (or introduction, etc.). This is where the author should indicate if the original records were incomplete or if there were issues or difficulties reading and transcribing the original record.
In some online databases, this information may appear in a "more about," or "frequently asked questions" page.
Not knowing what a book or database does not contain may cause you to think you searched something when you did not.
The preface of a book of marriage records from 1829-1900 may indicate that records from 1850-1860 are missing. That is something you need to know when using that publication. If you do not read the preface, you may never know.