There is one more problem you should be aware of.  All CDC devices (which include those "fake serial port chips") actually waste a huge amount of USB bandwidth.

The screen shot below shows typical data measured on a CDC "USB - Serial Bridge" chip, when the chip only idling: i.e. not transferring any data at all.  The measurement clearly shows that CDC devices waste 50% of the available USB bus bandwidth, even when they are not transferring any data at all.