100% duty cycle means that the fet will be on all the time, the current in the inductor, and therefore in the fet, will continue to increase and your fet will blow.
nominal duty cycles near the limits (close 100% for a boost and close to 0% for a buck) will work fine for a system that has a consistent load that does not fluctuate. problems will arise when the load increases - either slowly or quickly. more load requires larger duty cycle. a fast load step will require a sudden increase in duty cycle to keep the output from dropping out too much. parasitic board and component losses can also contribute to an increase in duty cycle (I*R drops will force higher duty cycles and as the load increases those I*R drops increase as well).
if the system does not have the headroom to accommodate for more duty cycle then the output will fall out, the duty cycle will go to 100% and unless there are protection features such as overcurrent or undervoltage that will shut things down, then the fet and possibly other components will fail.