I've used Digi devices for years, with few issues. Biggest system to date has four Etherlite 160's (total of 64 ports); sixty ports are tied to UUTs spewing 6400 char/sec each, all of which is digested by my LabVIEW application. The other four ports are used for instrumentation, doing query/response (a few dozen chars per message, as fast as the instrument responds).
These terminal servers are setup to use the Digi RealPort driver, which provides standard Windows comm API, so VISA treats them like local asynch serial. When you get up to this level of activity, with array-launched VI clones, and lots of queue/notifier/event structure support, you have to be pretty careful about the details. Seemingly minor changes to cloned code, reentrancy settings, execution system assignment, etc can mean the difference between a working app and a quivering heap of unresponsive code.
Dave