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.
From a customer standpoint, you're really interested in the I/O rate, data rate and consistency of performance. It may be a unified storage system that has enough performance for your application and that's great. If not, maybe a block-based system alone might be the best answer. Don't worry so much about the design details. Look at the system overall and determine if it will meet the needs of your applications. Performance variability often trips up customers, so be sure that the unified storage platform will provide adequate performance across a range of variability, and see if it is possible to dedicate resources to handle critical jobs. If you can manage that, a unified storage system may be a fitting solution.
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