With unisons, as with controllers, we are dealing with a historical problem. MIDI was developed without the idea that a channel might play more than one note at a time. And then, when multiple notes began to be played on a channel, the input controller was usually a keyboard. While a sustain pedal does make unisons possible with a MIDI keyboard, it still does not allow proper playing of unisons: any key number for which a note off message is more recent than the last note on message cuts off when you raise the pedal. If you merge two tracks that both contain sustain pedal messages, similar problems occur, and the results can be very unpredictable.