Scalability is a huge issue to consider. There are many ways to view scalability. For example, you might increase the number of storage systems or NAS filers in the environment. You might increase the number of file systems or the number of files. You might also increase the number of servers making storage requests or the number of I/O operations. Regardless of how you scale, the file virtualization platform should be able to maintain adequate levels of performance and availability.
Test the platform in your own environment and pose the hard questions to your vendors. Every vendor says they have an infinite number of supported devices or infinite capacity. That's common. But, press them for specific limits in terms of storage, file systems, connections and so on. What will they certify and support when you call for help? What will they support in writing?
Go to the beginning of the File Virtualization FAQ Guide.