Disadvantages of USB to RS232 dongles
Driver issues aside the main disadvantage of USB to serial converters ports that I'm aware of is increased latency. Some embedded devices, wireless modems, device programmers and the like often use the flow control lines such as DTR and CTS for non-standard purposes. They may not work if they are timing critical or transitions are required in between a sequence of data. I've encountered several devices like that, mainly older designs from when RS232 was more popular and always implemented as a standard UART.

Technically the USB driver stack also introduces some additional overhead and can cause some issues with old DOS applications. But neither of those is much of an issue with modern PCs and operating systems.