No, it's not the problem if all of the instruments support the command. The command is a GPIB standard. The vxi-11 and lxi protocols are what vendors use to provide the same basic functionality of gpib.
Of course not. The point I was making was the command set/features of a compliant Ethernet instrument is essentially the same as the vendor's gpib version. With VISA, you don't really care as the exact same code is used for both.