While it may be true that mega projects are by nature extremely difficult to estimate, another factor identified in the consistent pattern of estimating difficulties is:

another factor identified in the consistent pattern of estimating difficulties is: economic downturn and unidentified risks
Economic downturn refers to the fall of the nation's productivity as a whole that is caused by the decrease in real GDP and Unidentified risks refers to The type of risks that are not commonly present in a certain industry (usually caused by a changed in political climate or environmental damage)