A few of the factors that have to be taken into account:

People are injured and die. The number of people injured or dead during an evacuation has to be compared to the numbers that normally die or get injured every day in your average large city.

Moving the old, very young and the sick, mental or physical, eats up a huge amount of resources and even done with utmost care will lead to suffering and some percentage dying. Losses are unavoidable.

Some percentage, usually calculated as roughly 2% to 5%, will not leave willingly the threat of fire, flood, nuclear devastation, or certain death will not shift some people. What you do with or about these people remains an open question. Do you order the police or army to go house to house removing people? Can you afford the time and manpower? Even when it means getting into possible shootouts? Even when the end result will be to put you police at risk and the end result will still that some avoid the sweeps and remain behind?

A commonly used estimate of how many people can be moved in what time is that a single open lane will carry 1000 cars per hour and each will on average carry 2.5 people:

2500 people/lane/hour.

Under near ideal conditions ten lanes will move one million people in forty hours. Toss in a few flat tires, wrecks, cars running out of gas and your numbers go down from there.

Assuming you get your million people out, where do you put them. Providing even minimum levels of food, water, shelter, sanitation, security, and medical care for one million people is a serious challenge. Evacuate NYC and you have roughly 8.3 million people on your hands.