The biggest drawback is that appliances will not match other servers that are monitored and serviced by a vendor that you have established a trust with. I'm a big believer in finding one of the big three server vendors: HP, IBM, or Dell and sticking with them for support, monitoring software, etc. Even better if the SAN is the same provider as the servers. You may lose some performance per dollar spent, but you will have much better control over your environment.