I will throw my part as well .,
Like any spilt single phase circuits which it is typical used in North Americian side.
One example is on one side of MWBC is portable heater which it is 1500 watt then on other side of MWBC is your TV or computer which they useally draw about 200 ish watts and the netural is carry the differnce between the two loads.
Now let say someone which not aware this is MWBC and loosen up the netural for some reason instead of normal parallel split phase circuit it become series due the 240 volts will work it way around without netural so it will foruce 240 volts thru the example as I posted the heater itself will surive very easy but not for the computer or tv due the voltage for computer or tv will spike up anywhere from 140 to 240 volts depending on how well it designed.
That one quick prime example is 120 volt light bulb suddenly get super bright and burn out or explode ( I have see it happend from time to time but not often ) same thing with tv or computer or any appalinces with electronic control can get fried by overvoltage.
It will be the same idea with whole house system if you see large number of bright or dimmed light that usally indecated that you have comperised netural so anything on 120 volts will either get damaged or survived depending on the layout of circuits.
Oh by the way this not only affect the single phase MWBC's but it will do the excat the same thing with three phase MWBC's as well.
Let me post one example
one of our members in here do have wonderfull diagram so therefore Merci to Stubbie.,
Here the other one this will really clear it up alot .,
Merci,
Marc