Quoting from Medieval Genealogy.com:
At first sight, the heralds' visitations are an ideal source of information for the medieval genealogist. The visitations produced a collection of pedigrees of families with the right to bear arms, recorded between the early 16th and the late 17th century, but in many cases extending much further back. Though they are indeed a valuable source, they must be used with great care, and confirmed from contemporary records wherever possible.
This may sound too good to be true and sadly, in many cases, it is not true. While some of the heralds were pioneers in the systematic application of record evidence to genealogy, others were far less skilful and far less scrupulous. Oral information from the family, if unsupported by documentary evidence, can clearly be relied on safely for only one or two generations (or even less than this, to judge from some examples). Even where records were used, their evidence could sometimes be misinterpreted, and in only a few cases do the heralds present the evidence alongside the finished pedigree so that their deductions can be checked.
But worse than this, in an age where a distinguished descent was very desirable, the heralds seem often to have been willing to copy - or even to compile - long and impressive pedigrees which bore little relation to the truth. To make matters worse, later copies of visitation pedigrees often contain additions and continuations - these have sometimes found their way into print, because older editions were often based on copies rather than the original visitation books.
See:
http://www.medievalgenealogy.org.uk/guide/vis.shtml
