Miditech 4merge USB gets easily confused by my MIDI controller keyboard when I switch configurations (maybe it sends SysEx that ain't handled well or maybe it gets overrun by a flood of too many MIDI messages at once, I wouldn't know). Workaround: I keep the box near to me to reboot it (unplug-replug), thanks to the long MIDI cables I happen to own.
Otherwise, for the time I used it, it worked well handling usual MIDI messages (including but not limited to note off, note on (w/ velocity), poly touch (good!), expression).