The Ethernet data path (the red route) is now longer, it must travel through Network controller and USB hub controller. In order to develop realtime Network driver, we first have to build a realtime USB stack, the difficulty is now doubled. At least till this time, such kind of realtime network driver model is absent. This explains why I said almost current EtherCAT Master solutions for Raspberry pi are not hard-realtime.
Since it is impossible to build realtime Network driver for existing USB-Ethernet controller on Raspberry pi, the only possible solution to have realtime EtherCat Master on rpi is to use external NIC.
The next question is what interface of rpi we should use to communicate with the external NIC? Very fast answer, SPI: