It is incredibly easy to ensure that a controlled fire combusts all of its fuel completely. You just add excess oxygen to the combustion chamber.
Uncontrolled fires, and things burning under a blanket of ash, will not have sufficient oxygen in some places, will have CO as a reaction product instead of just CO2, and may leave unburnt carbon. Think about how charcoal is made--oxygen-free pyrolysis.
The waste product of a document incinerator is ash, water, and CO2. It is literally impossible to recover any information content from it.
Uncontrolled fires, and things burning under a blanket of ash, will not have sufficient oxygen in some places, will have CO as a reaction product instead of just CO2, and may leave unburnt carbon. Think about how charcoal is made--oxygen-free pyrolysis.
The waste product of a document incinerator is ash, water, and CO2. It is literally impossible to recover any information content from it.