If you open a /dev/ttyS0 no problem
if you open a /dev/ttyUSB0 mono runtime crashes with a strange error in
System.IO.Ports.SerialportStream.setSignal method it gives <filename unknown>:0
We use a modem connected to /dev/ttyUSB0 in a production environment,
previously using Mono 2.10 now using Mono 2.11 without any issues.
So this may be a localized issue very specific to your setup.