There are several issues. Caching difficulties are one concern since caching patterns may not be optimized for block-based storage. The other issue involves the mix of file and block functionality in the same platform -- mainly implementing block functionality on a NAS-based platform. Vendors are typically handling that integration with another layer of software (or two) that otherwise would not exist in a system designed specifically...
for block-based I/O. This results in a longer code path; impacting performance. That is, a native block-based storage system is almost always faster than a unified storage platform because the dedicated block system uses less software. Of course, that's comparing systems with equal hardware and back-end disks.
Listen to the unified storage FAQ audiocast.
Go to the beginning of the Unified Storage FAQ Guide.