In some ways a polygon boolean is not quite as difficult of a calculation, things tend to operate on a more granular smaller area.
NURBS booleans try to work with larger spans, larger areas and longer strings of intersection curves... One of the side effects of this is that if there is a difficulty in calculating one particular small area it can tend to mess up the entire boolean.
Of course when things work properly there is a lot of convenience in keeping larger sheets and strings of edges intact though.
 Michael
