I don't particularly want to lay blame on any one component, but I've been trying to set up some customer systems with three USB-UART virtual com ports, and it's not straightforward. The quality of the drivers is hard to discern, but WHQL certified drivers don't seem to help. The problem of wandering port numbers is very troublesome, and one significant issue is that many converters ship with the same USB serial number. Connected several devices with the same VID, PID and serial number does confuse Windows, and you will find warnings not to do it. If the converter uses the Silabs or FTDI chips you can at least serialise them yourself.