when you follow the procedure, things should work
there is a video demonstrating how the process looks like.. avrdude was used in the video, but it's pretty much similar when doing it over MIDI sysex, just with a few differences:
- via avrdude, by default there is verification - avrdude reads back the data after it has been written and compares it with the hex file
- via sysex - sending the data takes more time (because midi is slower), and the communication is just one way, so there's no way for the bootloader to tell if something went wrong or not
but basically, after flashing the firmware - the bootloader jumps to the firmware, and you should have the x0x "working"
if something went wrong - the firmware would probably be semi-corrupted, and then trying to run it would result in maybe faulty operation, but more likely a complete unresponsiveness.. but this rarely happens