According to my understanding NAS may result in security exposure, take a scenario for example
There is a user's requirement to upload document in Pega task. When the document is stored on NAS that is mounted on Pega Server, and an option can be made available to user to view the document from the NAS, Suppose we provide the location of the document as a link to share with other user or for other usability of document. Here in this situation, a secured document could face security exposure from unauthorised access when the link is passed to others.
On the other hand, PEGA OOTB table can be accessed based on authentication and authorisation configuration of Pega application.