Overvoltage protection after SMPS
Over volt protection is a common and accepted practice. There are many ways of doing this, but I won't mention them all. Firstly, you can place a big Zener diode across the output that is arranged to conduct heavily before the max voltage rating of the chips are exceeded. If the Zener eventually fails short circuit the chips are safe. 

Secondly, you can place a big SCR across the output that is arranged to fire when an overvoltage occurs. This is a very old scheme that dates back to big linear regulators and mainframe computers. 

Thirdly, what I do is place a cheap, low Rds(on) N-chan MOSFET in series with the output of the buck converter. I drive the gate of the MOSFET with DC that comes from the converter switching waveform. I use a simple diode pump to do this. If the main buck converter FET fails short circuit which is the most likely failure mode the diode pump stops and the power is cut off.

There are also converter topologies that don't give an overvoltage when the switch fails short circuit but they are not as simple and as efficient as the buck converter.