George Crump posted an article over on Network Computing discussing why storage is different for data protection. He makes a number of points regarding the benefits of using a storage appliance approach versus a software-only model, and for the most part, I agree with his analysis. However, there is an important point missing.
The software-only model relies on a generic software stack that can use any hardware or storage platform. This extreme flexibility also creates extreme headaches. The software provider or ISV cannot certify every hardware and environment combination and so the customer is responsible for installing, qualifying and testing their system. Initial setup can be difficult, but support can be even harder.
What happens if the product is not performing? The support complexities become difficult. Do you call your software ISV, your storage vendor, your SAN provider, your HBA vendor? There are a myriad of different hardware pieces at play and the challenge becomes how to diagnose and resolve any product issues. This is less of a problem in small environments with simple needs, and rapidly becomes an issue as data sizes grow.