Not true, actually. Some really special systems work afterwards. But not many.
In some ways, the Magic Smoke is good because it lets you see what's wrong. Better that than a dried up electrolytic. My A4000 has a cremated SMT resistor (the audio circuitry I think), a couple of bodged normal resistors, and black scorch marks - it works just fine. (It was a dodgy backplane that caused it, for those who are interested).
Don't give up on things which have let out the MBS, you can usually put it back in. Unless you do something really stupid like use the video cable as a lightning conductor. Then you're on your own.
