Because of the age of these censuses, their condition may not be as good as more recent enumerations, and it is more likely that fragments are missing. On the other hand, with the relative scarcity of other records for this time, this makes surviving censuses all the more valuable.
Many countries took periodic censuses to keep track of various aspects of the population. Where available, these records often include helpful details about your ancestors and their families.
While the questions in census records vary from place to place, and year to year, enumerations from the 1700s are generally leaner in content, but are nonetheless useful because they place your ancestor in a particular location at the time of the census. Knowing this you can branch out to nearby churches, cemeteries, and civil records that may include more detail.