The one main drawback with adding to many internal devices just like pci slot cards is the decrease seen in air flow along with more internal device drawing power. Usb devices run a far smaller current level where that is also something to consider power wise as well as heat wise.
Creative's Sound Blaster line of pci audio cards are noted for pulling on system resources when gaming or running multimedia. XP is also noted for resource sharing problems when more then one expansion card is used. Often the first pci slot has to be left unoccupied to correct this.