USB isn't really a serial bus
It has a lot more in common with twisted pair ethernet than it does with RS232 in that data is sent as packets over a wire and these paclkets are encoded/decoded by hardware into frame buffers before being pushed further up the stack. Its a lot more complex than just setting the DTR line to +/-5V and reading raw binary data off the wire as per old style serial buses and there's probably some hidden attack vectors in various USB implementations if anyone bothered to look (and I'm sure someone has);.