The advantages and disadvantages of using Geographic Information System (GIS) data that have been generated by people other than yourself, or institutions other than the one you work for are very similar to the advantages and disadvantages of using any other sort of information that you did not gather yourself. There is nothing that makes GIS data unique in this way.
When you get GIS data from an external source, you have an advantage because you have access to much more data than you otherwise would have. If you had to personally develop every bit of data that you used, you would be terribly limited. By using other people’s data, you have access to a broad range of information without having to gather it yourself. This is a major advantage.
However, when you use data that other people gathered, you are putting yourself at their mercy. That is, you are relying on them to have collected the data in a way that was competent. You have to trust that they gathered the data properly and that there are no glaring errors in the data that will cause problems for you if you use it.
Thus, using data from other sources allows you to have access to more data, but it forces you to rely on other people and hope that they have gathered and input their data properly, which will be true if the source is a reliable one, like NOAA's National Geophysical Data Center (NGDC) (www.ngdc.noaa.gov/).