Because of the vast number of commuters, a major problem facing modern megacities is:

Megacities have an impact on public health, like contamination of water. Overpopulated places can bring diseases and a lack of health protection.

Explanation:

Megacities are the base for gas emissions that could be very dangerous for the environment. Traffic jam is getting worse day by day. Almost half of the habitant of one city spends at least one hour in the car to get to work.

The answer is traffic and availability of the public transport vehicles.

This is based on my personal experience. Sometimes, I won't be able to take my own vehicle on my way to work. And this would leave me no choice but to commute myself to work. I should take a bus or subway or a cab. But on peak hours on weekdays, numbers of commuters like me would struggle to find an available public transport vehicles. Luckily, I could take either bus or a subway. But for others, these might not be available to them. Sometimes, major roads has so much cars in them that would result to extreme traffic. Bumper to bumper traffic are the worst. These are normal in megacities. This could result to waste of energy and time for the commuter.